P0171

System Too Lean (Bank 1)

Q: What does fault code P0171 mean?

System Too Lean (Bank 1). The ECM has detected that the Bank 1 mixture is too lean, with short-term and/or long-term fuel trim values reaching or exceeding the enrichment compensation limit (typically +25%).

Q: What are the possible causes of P0171?

Intake manifold leak (most common cause); MAF sensor dirty causing under-reporting of intake air volume; Low fuel pressure (fuel pump degraded, fuel filter clogged); Fuel injectors clogged; Oxygen sensor signal offset; Exhaust manifold or pipe leak.

Severity: Medium Fuel & Air Metering Generic Code

Detailed Description

The ECM has detected that the Bank 1 mixture is too lean, with short-term and/or long-term fuel trim values reaching or exceeding the enrichment compensation limit (typically +25%).

Possible Causes

Intake manifold leak (most common cause)

MAF sensor dirty causing under-reporting of intake air volume

Low fuel pressure (fuel pump degraded, fuel filter clogged)

Fuel injectors clogged

Oxygen sensor signal offset

Exhaust manifold or pipe leak.

Solutions & Repair Guide

First visually inspect all vacuum hoses and intake pipe connections; Clean the MAF sensor with MAF cleaner; Measure fuel pressure to confirm it is within specification; Check fuel injector spray patterns; Perform a smoke test to check for intake leaks. This is one of the most commonly encountered fault codes.

Disclaimer: The information above is for reference only and does not constitute professional repair advice. Actual causes may vary depending on vehicle make, model, year, and configuration. Always consult a qualified technician for accurate diagnosis.