What this code means

The ECM detected that the upstream O2 sensor is showing a persistent lean-biased signal with insufficient switching activity. The sensor is stuck or sluggish on the lean side.

Symptoms

• Check Engine Light
• Poor fuel economy
• Rough idle
• Failed emissions test

Common causes

• Aging O2 sensor with degraded response
• Exhaust leak before O2 sensor
• Vacuum leak causing actual lean condition
• Contaminated O2 sensor
• Wiring issue affecting sensor output

Diagnostic steps

1. Monitor O2 sensor signal switching with scan tool
2. Check for exhaust leaks
3. Perform smoke test for vacuum leaks
4. Inspect O2 sensor for contamination
5. Test sensor wiring

Repair notes

Replace the upstream O2 sensor if switching response has degraded.

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