P0040moderate severityPowertrain
O2 Sensor Signals Swapped Bank 1 Sensor 1 / Bank 2 Sensor 1
on 2019 Mazda Mazda3 2.5L I4 · FWD
What this code means
The PCM has determined that the upstream O2 sensor signals for Bank 1 and Bank 2 are swapped, likely due to incorrect wiring after sensor replacement.
Symptoms
- Check Engine Light
- Poor fuel economy
- Rough running
- Failed emissions
Common causes
- O2 sensor connectors swapped after service
- Wiring crossed between banks
- Incorrect sensor installation
Diagnostic steps
- Verify which O2 sensor is connected to which bank
- Trace wiring from each sensor to PCM connector
- Check for crossed connectors
Repair notes
Swap the sensor connectors to the correct banks.
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