Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: 2006 Pontiac Wave does not shift from park to drive.

Hi there. It sounds like the shifter interlock solenoid has failed in the shifter housing. Remove the shifter housing from the vehicle and test the solenoid for resistance. If there is high resistance or no resistance, then the solenoid has...

Q: Constant knocking sound from engine

Hello and thank you for contacting YourMechanic. The knocking sound is coming from the flex plate in the back of the engine. The flex plate is what drives the torque converter to the transmission. The flex plate is probably cracked...

Q: Loud Knocking Noise Coming From The Right Upper Side Under My Hood .

It sounds like you could be having a fuel delivery issue if your vehicle is not able to stay running. It is also possible that your engines knocking sound could be part of the reason it is dying. I would...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...