P0159: O2 Sensor Circuit Slow Response Bank 2 Sensor 2

What does P0159 mean?

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. P0159 means the downstream sensor on bank 2 reacts too slowly to mixture changes, typically measured around fuel cut-off. Thresholds are documented only for the bank 1 counterpart P0139 (Subaru: 0.51 s rich to lean, 2 s lean to rich); Jaguar Land Rover lists P0159 for its mid-cat sensor after water-induced thermal shock. Newer GM and Ford systems report direction-specific codes P013C/P013D and P014A/P014B instead. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on (usually after two drive cycles)
• Catalyst monitor readiness not set
• P0430 may follow

Common causes

• High Sensor aged or contaminated (oil ash, silicone, additives)
• Medium Exhaust leak ahead of the sensor
• Medium Sensor heater weak, sensor too cold
• Low Thermal shock damage from water

Likelihood is a general estimate across vehicles; your make and model may differ.