Q: Engine is running rough and P0401, P0171 and P0174 codes.

asked by on

Trying to diagnose codes listed above. Tested EGR for plugged or restricted lines. Checked OK. Will stall engine when idling and you cover EGR Control Solenoid vent with your finger. Pulled EGR valve and lines to visually check. All OK. All electrical checks of EGR Solenoid and DPFE are OK. Found vacuum to EGR valve keeping EGR valve open all the time engine is running. When tested with a vacuum guage you have 5 inches of mercury at idle which increases to 18 to 20 when you depress accelerator and drops back to 5 inches when you release accelerator. This happens even when truck is in PARK and engine is running. I think this is causing engine to act like it has a large vacuum leak which I think would cause the P0171 and P0174. I don't know what is causing the EGR Solenoid to allow the vacuum to pass through and open EGR Valve. Any advice? Sincerely, Ron Zara
My car has an automatic transmission.

The first thing to check is to see if the EGR solenoid will release the vacuum when the electrical connector is disconnected. If it still doesn’t release the vacuum then the solenoid should be replaced. The codes need to be cleared and the vehicle should be driven to see if any of the codes come back.

The EGR solenoid has a small foam filter on the vent side and it is there to keep dirt out of the valve. If dirt gets into it, the solenoid will be held open and allow vacuum to pass through it. If you find that the solenoid is not leaking vacuum when the connector is disconnected then you may have a problem with the DPFE telling the ECM to open the valve. From what it seems like, I think you will find the solenoid is going to be the problem. I’d recommend having a certified mechanic check your vehicle out, they will be able to properly diagnose your Check Engine Light codes and make the necessary repair.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 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!
  1. Home
  2. Questions
  3. Engine is running rough and P0401, P0171 and P0174 codes.

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: Loud cracking noise in the front when turning to the left or to the right, even on stationary.

This sounds like you either have binding sway bar links or arms. Have the sway bar checked and the links checked by putting hand on joint and having someone turn wheel. You will feel the creaking in the joint. If...

Q: Unable to go past 70-80 Km when going up a hill. Code P0174 with flashing engine light

Hello. A flashing Check Engine Light is an indicator that the computer has detected an issue that may potentially cause harm to the vehicle's emission system components. The code P0174 indicates a lean condition on bank 2 of the engine....

Q: Flashing CEL

The codes you recorded, P0304, P0171, P0301, P0300, all pertain to engine misfiring. The flashing of the Check Engine Light signifies that the misfiring is "active" (ongoing) and consequently you should try to get the car repaired before operating it...

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...
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...
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...