P0156 O2 Sensor Circuit Bank 2 Sensor 2

The oxygen sensor after the catalytic converter on bank 2 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. Bank 2 exists only on V and boxer engines with a separate downstream sensor per bank. P0156 is the bank 2 counterpart of P0136. Ford sets it as 'no activity' when the downstream sensor on bank 2 never swings outside a narrow band around 0.45 V in its functional check, if needed after forced mixture changes, once per drive cycle. Exhaust systems with a Y-pipe have only one downstream sensor, which counts as bank 1. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

Likely causes

Symptoms

• Check engine light on (usually after two drive cycles)
• Catalyst monitor readiness not set
• Usually no noticeable change in 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, fordservicecontent.com, static.nhtsa.gov.