P0139low severityPowertrain
O2 Sensor Circuit Slow Response Bank 1 Sensor 2
on 2021 Mazda Mazda3 2.5L I4 · FWD
What this code means
The downstream O2 sensor on Bank 1 is responding too slowly.
Symptoms
- Check Engine Light
- Failed emissions
Common causes
- Aging sensor
- Contaminated sensor
- Catalyst issue
Diagnostic steps
- Monitor sensor response
- Check catalyst efficiency
Repair notes
Replace downstream O2 sensor.
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