Diagnostic Trouble Code
P0171 — System Too Lean Bank 1
System Too Lean Bank 1
Definition
The ECM has detected that the air-fuel mixture on Bank 1 is running too lean — meaning there is too much air or not enough fuel. The long-term fuel trim has exceeded the positive correction threshold.
Common Causes
- Vacuum leak (intake manifold gasket, brake booster hose, PCV valve)
- Dirty or failing MAF sensor
- Weak fuel pump or clogged fuel filter
- Leaking or clogged fuel injectors
- Exhaust leak before the O2 sensor