This page is meant to be a reference to understand common diagnostic trouble codes that can cause your check engine light to come on and what things can cause each code.  If the suggestions we have don't help, its best to take your car to an experienced performance shop for further diagnosing.

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P0420 - Catalyst System Efficiency Below Threshold

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This code means your catalytic converter is not performing up to the ECU's required level.  If your down-pipe or J-pipe is stock, then it is likely it is simply worn out if your car has higher miles.  If you have an aftermarket catalytic converter, it might not be good enough to satisfy the ECU and might need to be replaced with something better.  Typically GESI catalytic converters can and will satisfy the ECU and keep this code away so we recommend only using aftermarket systems that use these.

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P0171 - Fuel System Lean

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This code means the fuel control system has applied the maximum amount of positive long term fuel correction that is possible and the ECU suspects there is a vacuum leak or other issue causing the need for this high level of correction.  Common causes include vacuum leaks, dirty MAF sensor or low fuel pressure.  Troubleshoot this by having a leak check done, cleaning the MAF and ensuring the car has normal fuel pressure.

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P0172 - Fuel System Rich

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This code means the fuel control system has applied the maximum amount of negative long term fuel correction that is possible and the ECU suspects there is a boost leak or other issue causing the need for this high level of correction.  Common causes include boost leaks or a dirty MAF sensor.  Troubleshoot this by having the car checked for boost leaks and cleaning the MAF sensor.

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P25b3-72 (GR Corolla)

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This code generally means you have developed a boost leak.  The ECU is seeing a mismatch in MAF input and boost pressure readings and generally the car will also not want to make any boost or perhaps not even want to stay running. Check for boost leaks, especially at the turbocharger where its common to have the charge pipe blow off.  Check all connections between the turbo and throttle body for any leaks.

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