P2A07 O2 Sensor Negative Voltage Bank 1 Sensor 2

Sensor 2 on bank 1 is the switching oxygen sensor behind the converter; P2A07 says its output has dropped below zero volts. Because the voltage comes from the oxygen difference across the zirconia element, a negative value points to reversed signal and return wires, a contaminated reference air path or a cracked element. No manufacturer documents P2A07 publicly. The closest documented test is Ford's: from 2011 a rear sensor held below −0.2 V for 10 s, put down to swapped wires or sensor degradation – Ford, however, files it as P2A01. A Denso patent watches for a negative output in overrun shut-off, a phase in which the exhaust behind the converter is almost pure air, to find element cracks. As long as the signal is negative, the converter cannot be assessed and the downstream trim switches off.

Likely causes

Symptoms

• Check engine light on
• Catalyst monitor of bank 1 not ready
• Usually no noticeable change while driving

Repair estimate

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

Parts involved

Related codes

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