P2A09 O2 Sensor Negative Voltage Bank 2 Sensor 1

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.

Likely causes

Symptoms

• Check engine light on
• Higher fuel consumption due to open-loop operation of bank 2
• Possible rough idle

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, patents.google.com, newtonnet.co.uk.