What this code means

The PCM detected a rich or lean cylinder-to-cylinder air/fuel ratio imbalance on Bank 1. The diagnostic monitors the pre-catalyst O2 sensor frequency and amplitude by calculating accumulated voltage over a sample period. An imbalance is flagged when multiple samples are consistently offset from the desired value. Very common on V8 engines with Active Fuel Management (AFM).

Symptoms

• Check Engine Light
• Rough idle
• Decreased fuel efficiency
• Engine hesitation
• Light or intermittent misfire
• Extended crank time
• Hard starting

Common causes

• Clogged or restricted fuel injector(s)
• Faulty upstream O2 sensor Bank 1
• Vacuum leak on Bank 1
• Dirty MAF sensor
• Cylinder misfire on Bank 1
• AFM (Active Fuel Management) lifter failure on V8 engines

Diagnostic steps

1. Check fuel trim values with scan tool
2. Perform injector balance test
3. Inspect for vacuum leaks with smoke test
4. Clean or replace MAF sensor
5. Test upstream O2 sensor Bank 1
6. Check for cylinder misfires

Repair notes

Check O2 sensor first if no performance symptoms. If running rough, inspect injectors for restriction. GM TSB #03-06-04-030F covers fuel injector clogging on 2000-2008 vehicles. On AFM engines, monitor for related misfire codes on cylinders 1, 4, 6, 7.

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