2015 Chevy Silverado P0171 and P0174 Lean Codes - Rough Idle

Question

My truck started idling really rough at stoplights and the check engine light just popped on. I checked with my scanner and it's throwing lean codes for both banks. It seems to run fine at highway speeds but feels like it wants to stall when I'm parked.

Greg Walters · Accepted Answer

The issue was the EVAP Purge Volume Control Valve located on top of the intake manifold. It was stuck wide open, sucking unmetered air into the combustion chamber at idle. I replaced it with a genuine GM part (PN 12668563) which cost about $45 and took 10 minutes to install. After clearing the codes and performing a fuel trim reset, the long-term trims dropped from +28% back down to a healthy +2%.

Pete Sanders · Answer

Could be a failing high-pressure fuel pump (HPFP) failing to maintain rail pressure, but usually that comes with a P0087 code. Since it's both banks, focus on things common to both sides like the intake plenum or the purge valve.

wrenchqueen · Answer

If your fuel trims improve when you increase the RPMs, you definitely have a vacuum leak after the MAF sensor. Check the intake manifold gaskets or the PCV hose connections near the back of the engine.

Sam Holloway · Answer

Take a can of brake cleaner or use a smoke machine to find the leak. On these EcoTec3 engines, the brake booster vacuum line is a common failure point that will cause a massive lean condition at idle.

2015 Chevy Silverado P0171 and P0174 Lean Codes - Rough Idle

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