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?

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: Hi hru, when hit a bump everything feels like it buckles underneath the car

Hello, thank you for writing in. The best thing to do in this situation is to get a full suspension inspection. Unfortunately this also means you need to inspect the tires, wheel assembly, brakes, and steering components as they are...

Q: Car whistling, rough idle.

This sounds exactly like the symptoms of a vacuum leak. When the engine inhales air through an opening it is not supposed to is a vacuum leak. The whistling sound is due to the air being drawn in through a...

Q: Intermittent failure to respond to throttle

Hello - wow, an older classic! Your 300TE utilizes Bosch Continuous Injection System (CIS) which varies fuel pressure more dynamically than more contemporary injection systems. I would suspect fuel pressure is the problem, both for the inability of the engine...

Related articles

P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.