2015 Chevy Silverado P0171 and P0174 Lean Both Banks - Rough Idle

Question

My truck started idling really rough at stoplights and the check engine light just popped on. It seems to run okay at highway speeds, but as soon as I slow down, the RPMs dip and it feels like it wants to stall. I plugged in my scanner and it is showing lean codes for both banks.

Lina Brooks · Accepted Answer

The issue was a cracked brake booster vacuum line where it connects to the back of the intake manifold. I performed a smoke test and found a heavy leak at the check valve grommet. Replacing the vacuum hose assembly (GM Part #22960339) and the booster check valve completely fixed the lean condition. Total cost was about $45 for parts and 30 minutes of labor; fuel trims are now back within +/- 3 percent.

Megan Rivera · Answer

Check your short term and long term fuel trims at idle versus 2500 RPM. If the trims improve significantly at higher RPMs, it confirms a vacuum leak. If they stay high, it might be a fuel delivery issue like a weak high-pressure pump.

Jordan Lee · Answer

Since it is affecting both banks, you definitely have a vacuum leak somewhere behind the MAF sensor. Check the intake bellows for cracks and listen for a whistling sound while the engine is running.

Carlos Diaz · Answer

Check the PCV tube that runs from the driver side valve cover to the intake manifold. These plastic lines get brittle and crack frequently on the EcoTec3 engines, causing a massive unmetered air leak.

2015 Chevy Silverado P0171 and P0174 Lean Both Banks - Rough Idle

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