Post Catalyst Fuel Trim System Too Lean (Bank 1)
Long-term fuel trim derived from the downstream oxygen sensor on bank 1 has reached the lean limit. The downstream-trim correction is a fine adjustment that runs after the main trim from the upstream sensor; persistent excursion suggests a degraded catalyst, an exhaust leak after the catalyst, or a drifting downstream sensor.
Affected components
- Oxygen sensor downstream
- Catalytic converter
- Exhaust pipe post cat
Common causes
- High likelihood: Exhaust leak after catalyst (post-cat sensor sees ambient air)
- Medium likelihood: Downstream sensor drifting
- Low likelihood: Catalyst lightly degraded
Possible symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Related codes
Sources
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.