2015 Chevy Silverado 5.3L P0171 and P0174 Lean Both Banks

Question

I am getting a check engine light with lean codes on both banks and the truck has a noticeably rough idle when stopped at red lights. The fuel trims are sitting around +20% but they seem to drop closer to normal when I am cruising on the highway. I don't hear any obvious whistling under the hood but something is definitely off.

Jordan Lee · Accepted Answer

The issue was a cracked PCV breather tube located at the back of the driver side valve cover. I replaced the plastic line with GM part #12675661. While I was in there, I found several intake manifold bolts were barely finger tight; I torqued them back to 89 lb-in following the proper sequence. Fuel trims are now back within +/- 3% and the idle is smooth as silk.

obd2_owl · Answer

Have you checked the PCV hose or the EVAP purge valve? If the purge valve is stuck slightly open, it will pull unmetered air into the intake and cause a lean condition primarily at idle.

backyard_bart · Answer

Take a can of brake clean and spray around the intake manifold gaskets while watching your short term fuel trims on a scan tool. If the numbers suddenly go negative, you found your leak point.

Marcus Bell · Answer

Since your trims improve at higher RPM/load, you definitely have a vacuum leak. Check the intake manifold bolts as these Gen 5 engines are known for them vibrating loose over time.

2015 Chevy Silverado 5.3L P0171 and P0174 Lean Both Banks

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