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?

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?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. After 'check engine' light, diagnostic revealed P0171 and P0402

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: Car accelerates when I'm not trying to

Hello. Your car uses a throttle body with a throttle cable to accelerate the engine. Most of the time this is caused by a throttle body that is dirty or failing or a throttle cable that is not releasing. It...

Q: Engine froze up

Hi there. If the engine will not crank over even with a fully charged battery, then there is another problem. If it is suspected that the engine is frozen then a mechanic would typically try turning over the engine by...

Q: Intake air flow leaking. What makes this happen?

Hi there. When having the code P0171, the fuel system is too lean. This could be the fuel pump is not working, the fuel filter is dirty or plugged, or the injectors are plugged or not working properly. Check the...

Related articles

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...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...