"Check Engine", "Service Engine Soon" light Questions and Answers

A: The code P0420 stands for "Catalyst System Efficiency Below Threshold Bank 1", which means the catalytic converter is faulty. The catalytic converter is installed in the exhaust and is an important part of the vehicle emission control system. To monitor how well the catalytic converter does its job, there are two oxygen sensors installed in the exhaust - one before and one after the catalytic converter. The engine computer constantly compares the signals from both sensors. If the catalytic converter can no longer do its job, the engine computer sets the P0420 code.
There could be few other reasons for the code P0420 to set, but in most cases, the catalytic converter will need to be replaced. This could be costly - the part is expensive.
If the car is still under emission warranty, or if it has expired just recently, your local Subaru dealer could help. If it's no longer covered by the warranty, any garage can do the repair. There might be even cheaper aftermarket catalytic converters available, but their quality is not always as good as the original one - I'd recommend to use the original Subaru catalytic converter.

A: The code P0171 reads "System Too Lean (Bank 1)". This means that the air-fuel mixture coming in to the engine is too lean, or there is too much air and too little fuel. This could be caused by a number of reasons, such as bad airflow sensor, vacuum leaks, low fuel pressure, clogged fuel injectors, etc. To know exactly what caused the problem, your mechanic can do a few tests.
From my experience, the airflow sensor problems are fairly common; sometimes the sensor just gets dirty and gives false air flow readings causing lean condition. Again, your mechanic can test it.
If the airflow sensor is causing the lean condition, it will need to be replaced. If the sensor is just dirty, your mechanic may suggest to clean it (cleaning the airflow sensor is very delicate procedure) as a temporary solution - sometimes this could work.

A: The code P0300 means "Random Cylinder Misfire". This means that the engine computer has detected that one or more of the cylinders misfire randomly. This code could take some time to diagnose, as there could be a wide range of reasons for the engine cylinders to misfire. Here are just a few examples: bad spark plugs, ignition wires, distributor cap/rotor, bad fuel injectors, bad airflow sensor, broken or weak valve spring, leaking valve, low fuel pressure, manifold gasket leak, clogged catalytic converter, etc. Take your truck to a knowledgeable mechanic and have it check out - driving with the misfiring engine can damage your catalytic converter.
The mechanic can scan the engine computer with the scan tool and see the conditions when the misfire happens and check which cylinders are misfiring. A number of of other things will need to be checked to find the cause (e.g vacuum leaks, exhaust back-pressure, compression, etc.).

A: The code P0300, "Random Cylinder Misfire" means that some of the engine cylinders misfire. The other codes just point which cylinders misfire: the code P0301 means the cylinder number one; the code P0302 means the cylinder number two and so on.
There could be a number of possible problems for these codes to set. For example, bad spark plugs, bad airflow sensor, fuel supply issues etc. The EGR system problems are not unusual on Honda V6 engines; problems with EGR system could also set this code. Another possible cause could be the valve adjustment - if the valves are too tight, the engine could misfire when cold. Visit your dealer or a mechanic and have your vehicle checked out - the catalytic converter could get damaged when the engine misfires.

A: The code P0401 indicates "Exhaust Gas Recirculation Insufficient Flow". The Exhaust Gas Recirculation system (EGR) is designed to reduce the amount of nitrogen oxides (NOx) in the exhaust emissions. Nitrogen oxides (NOx) are formed at very high combustion temperatures. The EGR system redirects some of the exhaust gases back into the intake manifold, which lowers the combustion temperature and consequently reduce NOx in the exhaust gases.
A number of problems can cause the code P0401 to set. For example, it could be sticky or clogged EGR valve or passage, bad EGR vacuum modulator or the Vacuum Switching Valve (VSV). Take your car to a dealer or a mechanic that can test the EGR system with the scan tool. It's fairly easy to find the cause of the problem with the proper tools and information. The repair shouldn't be very expensive.

A: The code P0442, "Evaporative System Small leak detected" means that the Evaporative system is not sealed properly and has a small leak. The evaporative system prevents gasoline vapors inside the fuel tank from escaping into the atmosphere. Most common cause for this code is when the fuel tank cap is not sealing properly. Check the fuel tank cap, make sure it's tight. If you think that it wasn't closed properly, the check engine light should go off by itself in a few days of driving. If the check engine light won't go off, there could be a number of other reasons, such as leaking ventilation valve, cracked charcoal canister, cracked or pinched hose, leaking seal, etc.
If the leak is very small, it could be very hard to find. Take your car to a dealer or well-equipped garage that can test your evaporative system with the special equipment.

A: The code P0116 reads "Engine Coolant Temperature Circuit Range/Performance Problem". This code usually points to faulty engine temperature sensor in Volkswagen, however, few other things, such as bad thermostat or water pump could also set this code. A quick look at the freeze frame for this code can point to the right direction. A freeze frame is stored in the engine computer at the time the fault is detected. The freeze frame can be accessed with the scan tool. It contains data such as engine temperature, vehicle speed, engine rpm, etc. at the time when the fault occurred.

A: The code P0305 means the cylinder 5 misfires. There could be many reasons for one particular cylinder to misfire, but first, I'd check if the spark plugs have ever been changed on this car. With this mileage it's very possible the spark plugs are still original and they could be pretty worn by now.