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Q: After 'check engine' light, diagnostic revealed P0171 and P0402

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After my 'check engine' light came on, I went to the car shop and the mechanic's diagnostic said codes P0171 and P0402. What do these mean and can you tell me what to do?

A: The first code, P0171, is an indication the...

The first code, P0171, is an indication the engine is running lean. This code can indicate anything from clogged air and fuel filters to problems with O2 sensors and mass air flow sensors to intake gaskets.

The other code, P0400, indicates that the exhaust gas recirculation (EGR) system is not working correctly.

Both of these codes only give you a tool to help in the diagnosis of the problem; several checks to properly diagnose the problems would need to be done with a Check Engine Light inspection by a mechanic, such as one from YourMechanic.

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