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

Question

My truck is idling really rough and the check engine light just came on while sitting at a red light. It feels like it wants to stall out but runs okay once I get up to highway speeds. I hooked up my scanner and I am seeing lean codes for both banks.

Mike Reynolds · Accepted Answer

The issue was a cracked PCV breather tube located at the back of the driver's side valve cover. I used a smoke machine and saw it leaking heavily from the plastic connector. I replaced the hose assembly (GM Part #12639611) which cost about $25 at the dealership. After clearing the codes and performing a fuel trim reset, the idle smoothed out and the fuel trims returned to +/- 3%.

Kevin Tran · Answer

Check your Long Term Fuel Trims at idle versus 2500 RPM. If the trims improve significantly at higher RPMs, you definitely have a vacuum leak somewhere behind the MAF sensor.

obd2_owl · Answer

Don't rule out a dirty MAF sensor. Grab a can of dedicated MAF cleaner and spray the wires down. Sometimes oil from aftermarket filters gunk up the sensing elements and cause false lean readings.

wrenchqueen · Answer

These EcoTec3 engines are notorious for the PCV hose cracking or the oil fill cap seal failing. Also, take a close look at the evap purge valve; if it is stuck open, it will pull unmetered air into the intake.

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

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