Skip the auto shop - Our mechanics make house calls

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.

A: The first thing to check is to see if the E...

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 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

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: Can I use 15% ethanol fuel?

Any "conventional" light duty vehicle manufactured in model year 2001 or later is qualified to use 15% ethanol fuel (E-15). And, of course, flex fuel vehicles can use E-15 as well. Since your vehicle was built for model year 2009,...

Q: Still getting error codes

The codes you have for your GMC Yukon, p0171 and p0174, are informing you that the engine is running lean. This means that there is unmetered air entering the engine after the mass airflow sensor and is disturbing the air/fuel...

Q: My SUV is running hard and has the Diagnostic Trouble Code P0016

Hi and thanks for contacting Your Mechanic. The code P0016 is for the crankshaft position sensor. Check the wiring harness to the sensor that is located on the bottom of the engine by the crankshaft pulley. Check for voltage and...

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...
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...