P2A09O2 Sensor Negative Voltage Bank 2 Sensor 1
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P2A09 concerns the pre-converter oxygen sensor on bank 2, so it only arises on two-bank engines that use switching sensors ahead of the converters. The module reads a voltage below zero from it. Since a zirconia cell's output depends on the oxygen difference between exhaust and reference air, a negative value means swapped wires, a reference air path fouled by fluids or sealant, or a crack in the element; a Denso patent identifies such cracks by a lasting negative output in fuel cut-off. No manufacturer documents P2A09 publicly; Ford, for instance, records negative readings of its rear sensors under range/performance codes instead. P2A06 is the bank-1 counterpart. Bank 2 then loses its mixture feedback and may run in open loop, with higher consumption.
- What you may notice
- Check engine light on
- Higher fuel consumption due to open-loop operation of bank 2
- Possible rough idle
- Likely causes
- LikelyCracked element in the bank 2 front sensor
- PossibleSignal wires swapped or bank 1 and bank 2 connectors interchanged
- RareReference air path contaminated by oil, water or sealant
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €350
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.