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Old 17-04-2017, 02:14 PM
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OBD Codes Explained

OBD codes can confuse pretty much everyone...even mechanics. Just because you have diagnostic codes for a certain part on the car, it does not mean that that part needs to be replaced. It means that this SYSTEM is having a problem and it needs further diagnose to determine why the computer is setting the code.

This problem is made worse by garages that are trying to sell parts. Some of them (definitely not all) will try to get you to buy the part that they think is the problem simply by reading the diagnostic codes.

That being said we can still advise you to go to your local garage to get the codes read unless you have a great local mechanic that you know and trust. Garages will usually read them for free. The key is to not let the guy at the garage talk you into buying any parts before he can figure out what is wrong with your car.

After you know the code you can come back to this site and look up the meaning as well as common causes and diagnostic aids. On the next few posts of this section you will find the most common OBD codes and the things that commonly cause the problem. Let's take a look at what OBD codes actually are.


Different Types of OBD Codes Explained

Most codes that you will see are "generic", usually starting with P0xxx, but there are also codes called "manufacturer codes". Information about them is harder to find because these are more specific.

If you are looking for manufacturer specific codes, we recommend you turn to a good online repair manual.

To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.

With step by step repair instructions and detailed diagrams beyond what is found in most printed manuals, you can check this site or ask for diagnostic and repair information and help.

Car sensors are responsible for generating auto diagnostic codes.


Understanding Check Engine Light Codes

This information should work for any vehicle built from 1996 to today (there will be exceptions though).

Example P0340 (Camshaft sensor code)
1st Digit: B is for Body, C is for Chassis, P is for Powertrain, U is for Network
2nd Digit: 0 is for SAE aka generic, 1 is for mfg (Manufacturer Specific Codes)
3rd Digit=

* 1 Fuel and Air Metering
* 2 Fuel and Air Metering Injector Circuit
* 3 Ignition System (Including Misfires)
* 4 Auxiliary Emissions Controls
* 5 Vehicle Speed Controls And Idle Control System
* 6 Computer Output Circuit
* 7 Transmission
* 8 Transmission

4th & 5th Digit - two place fault code 0-99


Types of DTC

There are two types of obd codes (DTC) that apply to OBD II. I have them listed below with Type 1 being the more important because they can cause quick and severe damage. Type 2 being not quite as important but you still need to deal with it.

Type 1

1. Emissions related (things like EVAP codes).

2. Illuminates the MIL after one "driving cycle" that has failed.

3. Creates a trouble code freeze frame (information about other things that were going on in the engine) after one "driving cycle" that has failed.


Type 2

1. Emissions related OBD codes that aren't causing lost of pollution.

2. Will set a "pending" trouble code after one failed driving cycle.

3. Will clear a "pending" trouble code after one successful driving cycle.

4. Turns on the check engine light after two consecutive failed driving cycles. Stores a freeze frame after two consecutive failed driving cycles.


OBD Codes Categories

P0100-P0199 – Fuel and Air Metering. These will be things like the mass airflow sensor and throttle body

P0200-P0299 – Fuel and Air Metering (Injector Circuit). These codes would be for things involving the fuel injectors.


P0300-P0399 – Ignition System or Misfire. If there was a bad spark plug or faulty coil you would see one of these codes.

P0400-P0499 – Auxiliary Emissions Controls. These are the famous EVAP system codes.

P0500-P0599 – Vehicle Speed Controls and Idle Control System. The idle air control valve and VSS would fall under this category.

P0600-P0699 – Computer Output Circuit. Sometimes these types of codes point to a faulty computer.

P0700-P0999 – Transmission Codes. Many times there is a code set in the engine computer that simply lets the driver know that there is a fault in the transmission computer and the TCU needs to be scanned for codes.

Now you have a better idea of what makes up a DTC (Diagnostic Trouble Code). You can see that every digit has it's purpose. You also know which system is having the problem just by knowing what the first few numbers are. Let's take a look at some specific codes and find out what the likely fixes are.

Last edited by Holly Goodhead; 18-04-2017 at 01:44 PM.
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Old 17-04-2017, 02:46 PM
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Check Engine Code P0171

Code P0171 "system too lean bank 1" means that the computer has noticed (by reading the oxygen sensors) that there is too much air going into bank one (the bank that includes cylinder number one in the firing order in v style engines).

This information is also applicable to code P0174, system too lean bank 2, as this code is the same except of the opposite bank of cylinders.

Engine control computers need to have a specific ratio of air to fuel in order to work properly, and if there is too much of one or the other then it will set a code.


What Causes a P0171 Code?

Dirty Mass Airflow Sensor
Vacuum Leak
Bad Oxygen Sensor(s)
Low Fuel Pressure
Running Out of Gas
Less Common Causes
Clogged Fuel Injectors
Leaking Exhaust System
Evaporative Emissions System Malfunction
EGR System Malfuncion


How to Fix Code P0171

1. Check Gas Level
The first thing that I would do is make sure no one has ran out of gas in the vehicle recently. This will cause the P0171 code to appear almost every time.

2. Clean MAF Sensor
If you decide to clean the MAF yourself you need to be very careful as it is very fragile. There is a small wire that you will be cleaning that is very easy to break. It is best to use just the aerosol mass airflow sensor cleaner as using a q-tip or something like that can leave debris on it and cause it to not read correctly.

Then you should check for vacuum leaks, especially ones that are after the MAF like tears in the tube that goes from the airbox to the engine. If this is torn it will let air in that the computer cannot tell is coming into the engine.

3. Check Oxygen Sensors
The next most common cause for this code code is bad oxygen sensors. If they are not working properly then they will not be reading the amount of oxygen in the exhaust gas.

4. Check Fuel Pressure
Another common cause of this code is low fuel pressure. If the fuel pump does not put the right pressure of fuel into the fuel system then the result can be this OBD code.

If you want to check it you will need to have a fuel pressure tester that will fit your fuel system. Then you just need to check the pressure. If it is low you need to replace the fuel filter first (unless it has been done recently). After the fuel filter has been changed you need to check the fuel pressure again. If it is still low, then you most likely have a fuel pump problem.

These are the easier things to check with code P0171. If none of this fixes it then it would be a good idea to take your car to a professional.

Last edited by Holly Goodhead; 17-04-2017 at 02:52 PM.
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Old 17-04-2017, 02:56 PM
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Code P0300 - Causes and Fixes

Code P0300 is a code that is generated when your engine has a misfire that is not related to only one cylinder.

Sometimes this code will be set in addition to other codes such as P0301 which means that there is a misfire on cylinder 1 (cylinder 1 is the number one cylinder when looking at the firing order. It is generally the forward most cylinder on the engine.). If these engine codes are present as well the you should diagnose those first as they are probably causing this trouble code.

This code will set your check engine light and needs to be fixed as soon as possible. If it is not then you will probably need to replace your catalytic converter as well as the problem that is causing your misfire.

There are many causes for this OBD code. Since it is a "random misfire" or "multiple misfire" this means that the misfire is on different cylinders and not just one all the time.


What Causes a P0300 Code?

Low Fuel Pressure
Vacuum Leak
EGR system malfunction
Internal Engine Problems (such as low compression)


Less Common Causes

Faulty Coil
Faulty Spark Plugs and/or Wires
Camshaft or Crankshaft sensors
Ignition module
Computer
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix Fault Code P0300

This code is one of the most common codes ever. If you notice that your check engine light is on you should have it checked for this code. If it is flashing then it is almost certainly this one or one like it such as P0301, P0302, etc. These are codes that show which cylinder the misfire is located in.

The first thing I would do if the problem wasn't obvious by inspection is a full tune up. Many times this will fix this code. If you want to do more diagnosis so you don't just throw parts at it you will need to check each of the above items starting with the top four.

The two most useful tests for diagnosing this code are a fuel pressure test and cylinder leak down test.

Testing your vehicle fuel pressure is simple - All you need is a good fuel pressure tester, and care it doing it, remember fuel is under high pressure.

A cylinder leakdown test is a little harder to do for the DIY mechanic because you need a source of compressed air. If you have that though, you can just go buy a cheap leakdown test kit. This tool simply puts air into the cylinders when they are at top-dead-center (they should be sealed at this point). If there is leakage at this point then you will have a low compression problem, and need to find where the air is leaking from.

If you don't know how to check the items in the list above, you should take your car to a good garage. It probably will not (in most cases) cost too much for them to repair this problem. You car will run better, have more power and get better fuel mileage if you get the P0300 code repaired. It will also not be causing other problems by driving it like this ans risk cat damage.

Last edited by Holly Goodhead; 17-04-2017 at 03:14 PM.
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Old 17-04-2017, 03:20 PM
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Check Engine Code P0301-2345678

Are you struggling with check engine light code P0301? Learn what causes this fault code and how to fix it.

Troubleshooting code P0301 (and other P030X codes such as P0302, P0303 etc.) is easier than most would expect because your vehicle's computer has already figured out which cylinder that is having problem... saving you a lot of diagnostic headache.

Misfire codes (P0302, P0303, etc.) are some of the most common OBD codes. They basically mean that there is a misfire on that respective cylinder in the firing order. For example, P0306 means "misfire cylinder 6", P0302 means "misfire cylinder 2" and so on.

If your car has one of these codes you need to get it fixed asap. If you choose to drive your car in this condition you will do damage to the catalytic converter.

Driving with a misfire causes unburned fuel to be put into the exhaust (because there is one or more cylinders that are not burning it like they should) which then is sent to the catalytic converter. It is not made to clean up exhaust with unburned fuel in it and will soon become faulty. This is an expensive repair so the best thing is just to get your car repaired! Let's see what are common causes and fixes for this trouble code:


What Causes a P0301 Code?

Spark Plugs and Wires
Fuel Injectors
Internal Engine Problems such as Low Compression, etc.
Less Common Causes
Faulty Computer
Coil or Coil Packs
Crank or Cam sensors or reluctor wheels
Faulty O2 (Oxygen) Sensors
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix Engine Code P0301

1. Inspect the Problematic Cylinder
The first thing to do if you have this code is to inspect EVERYTHING that has to do with that specific cylinder. Since the misfire code is for that cylinder only it has to be only something that would affect that cylinder and not the others. This is the key to diagnosing this problem.

You want to be looking at the spark plug, wire, coil and injector of that cylinder. You also perform a compression test with a compression tester kit... followed by a leak down test using a leak down tester to see if your piston rings or cylinder head valves are getting worn and causing low compression.

A good way to check the plugs, wires and fuel injectors is to switch them with different cylinders to see if the misfire moves with them.


2. Swap Parts with Other Cylinders

Some of the easiest ways to diagnose this OBD code are to change parts. In order to figure out if the check engine light code P0301 (misfire on cylinder 1) is being caused by a fuel injector, you can (in most cases) just switch it with a different cylinder.

So if you switch the fuel injector from cylinder #1 (because of code 0301) with cylinder #3 you can then start the vehicle and let it run. When the check engine light comes back on you just read the codes again. If the code that comes back is p0303 then you know that the problem is the injector and you need a new one.

This method works for lots of things. You can use it with spark plug wires, ignition coils (assuming you have a coil for each cylinder), spark plugs, etc. If you have a coil on plug system (COP) then you can just switch the whole assembly with another one. This diagnosis method works for any P0xxx code.

Remember this type of code is one that cannot wait long for you to attend to. You will likely notice lower fuel mileage, loss of engine power, blinking check engine light and probably others too. Since there is a misfire, that means that raw fuel is going into your catalytic converter. The CAT is not made to process raw fuel. It will not take long for the CAT to go bad and if it does you are looking at big repair bill.

Last edited by Holly Goodhead; 17-04-2017 at 03:30 PM.
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Old 17-04-2017, 03:36 PM
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Check Engine Code P0340

What are the causes of check engine light code P0340 and how do I fix it?

This is a fairly common fault code. Your vehicles computer (PCM, ECM, etc.) uses the CMP (camshaft position sensor) to figure out what it should set the timing at. This is a very important thing because without knowing what the timing should be the computer is useless.

This code basically means that the computer has sent (or thinks it has) a signal to the camshaft position sensor, but it does not see the correct signal being returned from the sensor. Some symptoms that you might have noticed from this code are hard starting, loss of power, engine popping noises, misfires, check engine light etc.


What Causes the P0340 Code?

Camshaft Position Sensor
Wiring Problem
Less Common Causes
The Computer
The Reluctor Wheel on the Camshaft
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix Engine Code P0340



The first thing you need to check is all of the wiring you can see. Start with the battery cable connections make sure they are clean and tight. This might seem like a dumb thing to do, but if the computer does not get a steady feed of electricity from the battery it can cause all kinds of problems.

Check for any wires that might have rubbed through or any connectors that might be loose or have come disconnected. Check the fuse box in the vehicle and the relay center under the hood. Make sure the fuses are good and there is no melted plastic, indicating a fire. These are the main wiring problems you are looking for.


If you have checked all of the obvious things and still can't find the solution, then you have to start suspecting either the sensor or the computer.

Camshaft position sensors go bad a lot more than PCM's do so I would suspect that first. Unfortunately there is no easy way for you to test it. It will take some expensive test equipment and a skilled technician to test the sensor itself.

If you are a risk taker, you can just buy the CMP sensor and install it. It might fix the problem but there is no guarantee without properly diagnosing it.

Last edited by Holly Goodhead; 17-04-2017 at 03:41 PM.
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Old 17-04-2017, 03:45 PM
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Check Engine Code P0401

What does P0401 code mean? Find what causes it and learn how to fix this check engine code.

This trouble code "insufficient EGR flow" means that there is not enough exhaust gasses flowing back into the incoming air to the engine. EGR stands for "exhaust gas recirculation". The EGR system consist of three parts: EGR valve, DPFE (differential pressure sensor EGR) and actuator solenoid.

Car manufacturers created this system so already burned exhaust gas can flow back into the engine (when the EGR valve is open) and lower the combustion temperatures. This helps reduce emissions.

This OBD code basically means that the cars computer has tried to open the EGR valve, but it cannot get enough exhaust gas into the engine. EGR systems used to be controlled by engine vacuum via hoses and valves. Now most of them are controlled by electrical solenoids.


What Causes the P0401 Code?

EGR Valve
Blocked EGR Passageways
EGR Control (No or Low Vacuum)
Faulty solenoid or wiring
Less Common Causes
The Cars Computer
DPFE (Delta Pressure Feedback EGR) Valve
Modulator Valve
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix Engine Code P0401

Each car manufacturer has their own EGR system design but the most likely two causes of this code are the valve itself or passageways (many times the intake manifold will have very small passageways that get clogged with soot) that the exhaust gasses pass through.

Many times this code is caused by clogged EGR passages, and sometimes removing the sensor will give you access to the clogged EGR passages.

Another thing that you can do yourself is to check all vacuum hoses and all wiring going to the EGR valve. You might find a hose with a hole in it or you could find a connector that is not seated all the way or some wiring that has been rubbed through.

This fault code is a tougher one for a DIY mechanic to diagnose unless you have good experience and some specialized tools. Many times this code will go away if you replace the EGR valve (but not always). If you lack the skills or patience to diagnose this problem, you can consider giving it a shot and just replace the EGR valve.

Last edited by Holly Goodhead; 17-04-2017 at 04:05 PM.
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Old 17-04-2017, 04:10 PM
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Check Engine Code P0420

P0420 code "catalyst system efficiency below threshold bank 2" is a code that your cars computer will set when it sees that the catalytic converter is not working efficiently. This can be either because the CAT (Catalytic Converter) is not working properly, but it is more often set because of something besides the CAT.

What Causes This Code?
Engine Misfire
High Engine Oil or Coolant Consumption (Being burned in the cylinders)
Rich or Lean Fuel Mixture
Oxygen Sensor
Less Common Causes
Retarded Spark Timing
Leaded Gasoline Used
Leaking Exhaust System
Malfunctioning CAT
Weak or Poor Spark
Coolant Temperature Sensor
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix Code P0420

The first thing to do when trying to diagnose this code is to diagnose any other code that might be present. It is unlikely that this code will be set by itself.

If there are no other trouble codes, then check your engine oil and coolant levels first. If they are low, it indicates that they are either leaking externally or being burned in the engine. The next step to check are the oxygen sensors. If they are not working properly, they can also cause this type of code.

If it has been a while since your vehicle has had a tune up, then you might want to consider giving it one. Especially if you have noticed it running rough or the check engine light flashing. Engine misfires are sometimes a cause of this problem and by giving your car a tune up you will reduce the likelyhood that your engine will have a misfire.

Last edited by Holly Goodhead; 17-04-2017 at 04:13 PM.
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Old 17-04-2017, 04:17 PM
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Code P0441 - Causes and Fixes

Check engine code P0441 "Evaporative Emission Control System Incorrect Purge Flow" is set when the computer sees that there is not enough flow from the purge valve.

Basically this means that there is a problem in the EVAP system (for more information on this see my write up on code P0440). The computer will command the purge control valve open and if it does not sense the right amount of vacuum then it will set this code.

The purge valve is part of the EVAP system. The purpose of this system is to take all of the excess fuel vapor from the gas tank and store it until it can be burned in the engine. If it weren't for this system then all of those vapors would be vented into the atmosphere. Let's find out what are the main culprits of this OBD2 code:


What Causes Code P0441?

Faulty Purge Control Solenoid
Faulty Purge Control Valve
Damaged Wiring to the Purge Solenoid
Less Common Causes
Faulty Computer
Restriction in the EVAP System
Faulty Vacuum Switch
To troubleshoot, repair and maintain your vehicle, you'll need diagnostic and repair information that is specific to your car or van.


How to Fix a P0441 Code

The first thing to check is for any hoses that might be damaged or disconnected under your hood. These hoses are the small black ones and should all be connected to something. Check to be sure that they are all connected to something and that they are all routed away from the exhaust manifold and any other place that they might rub. This is not a common cause but you might as well check it since it is easy to do.

You can also check the wiring to the purge solenoid and the connector to make sure they are all in good condition. You probably won't find the problem by checking this but it is worth a try.

Other than that there is not much else that the average car owner can do to diagnose this error code. The diagnosis that would need to be done would include testing to see if the computer is commanding the purge control solenoid open and how much voltage is on the command wire. Then testing to see if the solenoid is opening correctly.

Last edited by Holly Goodhead; 17-04-2017 at 04:22 PM.
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Old 17-04-2017, 04:25 PM
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Check Engine Code P0442

What does P0442 code mean? "Evaporative Emission Control System Leak Detected (Small Leak)" - In a sentence, this is the code recorded in the your car's computer when it sees a small to medium sized leak in the system. Since your evaporative emissions system (You can read more about this at my tutorial for code P0440) is comprised of long hoses, there are many places that it can leak.

What Causes This Trouble Code?
Fuel Cap
EVAP Hoses

How to Fix Engine Code P0442

The first thing you will want to do is tighten or replace your fuel cap. This is the most common cause of this code.


If that does not fix it then you should open your hood and inspect all hoses for damage.

If you do not find the problem with these methods then it would be best to take your car to a shop that has a "smoke machine". This machine introduces smoke into the system and then it will come out of the same place that the system is leaking. This is BY FAR the best way to fix this problem.
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Old 17-04-2017, 04:28 PM
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Check Engine Code P0455

The P0455 code basically means that there is a very large leak or no purge flow from your EVAP system (to learn more about this system please read my write up about code p0440 and p0441). When the computer sees that the fuel fumes from the EVAP system are not flowing into the engine when they should be it sets this trouble code.

What Causes This OBD2 Code?
Fuel Cap Loose, Missing or Defective
Fuel Pipe Damaged
Less Common Causes
Fuel Tank Pressure Sensor Malfunction
Fuel Vent Assembly Blocked
Plugged EVAP Hoses
Accessories Not Conforming to EVAP Specifications

How to Fix Engine Code P0455

There are not too many things that can cause this code because there are not too many things that can cause this large of a leak.

If you have this code the first thing you should do is check your fuel fill cap and the fuel fill pipe. You might even want to just replace the fuel cap as it is very likely that it is what is causing this problem. If neither of those things fix it, then you should open your hood and look for missing or damaged hoses.

If you still cannot find the problem then you should take it to a shop that has a "smoke machine". They can introduce smoke into the EVAP system and see exactly where it is leaking from.

If you have already tried the above steps and still can't find the leak, then it is unlikely that you will find it. If that is the case, asking a professional mechanic or taking your car to a trusted repair garage will be the best way to go.


Disclaimer

This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. The Sixties Garage accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Last edited by Holly Goodhead; 17-04-2017 at 06:40 PM.
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