P2A08 O2 Sensor Negative Voltage Bank 1 Sensor 3

P2A08 is the negative-voltage fault of the third oxygen sensor on bank 1, which sits behind a second converter or another aftertreatment stage. Such a sensor is normally a zirconia switching type whose output should stay above zero; an intact, correctly wired sensor does not deliver a negative reading in normal operation, so the code suggests swapped leads, a short between the signal return and another circuit, or a cracked element. No manufacturer documents P2A08 publicly; this description rests on CARB's duty to catch open circuits and readings outside the valid band, on Ford's −0.2 V limit for rear sensors (stored there under range codes) and on a Denso patent for crack detection. Sensor 3 hardly influences mixture control, so the main consequence is an incomplete converter monitor.

Likely causes

Symptoms

• Check engine light on
• Converter monitor readiness missing
• Normal driving behaviour in most cases

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.