Quick checklist
Inspect intake boots clean MAF with MAF-safe cleaner smoke-test for vacuum leaks
OBD2 Codes
P0171 – System Too Lean (Bank 1)
Air/fuel mixture too lean on bank 1; commonly vacuum leaks or a dirty MAF
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Details
The ECM detects lean trims on bank 1. Start with intake leaks and MAF contamination before replacing O2 sensors.
Typical causesVacuum leak dirty MAF weak fuel pump low fuel pressure
SymptomsRough idle hesitation MIL on higher fuel trims
SeverityMedium — fix soon to protect cats and drivability
DIY checksInspect intake boots clean MAF with MAF-safe cleaner smoke-test for vacuum leaks
ToolsOBD2 scanner with live fuel trims fuel pressure gauge
Utilities
Copy-paste helpers
Related
Other pages in OBD2 Codes
FAQ
Common questions
Should I replace O2 sensors first?
No—verify vacuum leaks and MAF readings before swapping sensors.