Main Catalyst Efficiency Below Threshold (Bank 1)
Catalytic converter efficiency is below threshold, indicating reduced emissions control capability. Engine may be running rich or the converter itself may be degraded.
What You Might Notice
- Check engine light illumination
- Increased tailpipe emissions
- Possible reduced fuel economy
- Rotten egg smell from exhaust
- Engine running rough or misfiring
Most Common Causes
- 1
Faulty catalytic converter
Converter substrate degradation or internal damage reducing catalyst efficiency.
- 2
Rich fuel mixture
Oxygen sensor or fuel injector issues causing excessive fuel in combustion.
- 3
Failing oxygen sensor
Upstream O2 sensor not accurately reading exhaust gases for proper fuel trim.
- 4
Engine misfires
Unburned fuel entering converter and damaging catalyst material.
How to Diagnose It
- 1
Check Faulty catalytic converter — Converter substrate degradation or internal damage reducing catalyst efficiency.
- 2
Check Rich fuel mixture — Oxygen sensor or fuel injector issues causing excessive fuel in combustion.
- 3
Check Failing oxygen sensor — Upstream O2 sensor not accurately reading exhaust gases for proper fuel trim.
- 4
Check Engine misfires — Unburned fuel entering converter and damaging catalyst material.
How to Fix It
- 1
Replace catalytic converter
Install new OEM or quality aftermarket converter to restore emissions control.
- 2
- 3
- 4
Diagnose engine misfires
Replace spark plugs or coils if misfiring is causing converter damage.
Need a deeper diagnosis?
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