P0159 — Oxygen sensor circuit slow response — bank 2, sensor 2

P0159 indicates a signal response slower than the monitor expects for bank 2, sensor 2. Sensor 2 is usually after the first catalyst; some exhaust layouts also have sensor 3. Slow response differs from complete absence of activity.

Affected system: Powertrain

Symptoms

• Engine warning lamp and a stored code.
• Driveability symptoms are often absent; emissions inspection issues may occur.

Possible causes

• Aged or contaminated monitoring sensor.
• Inadequate heating, an exhaust leak or mixture conditions disrupting the test.

What to check

• Confirm B2S2 in the engine diagram and save related codes and freeze-frame data before clearing anything.
• With the exhaust cold, inspect the B2S2 harness for melted insulation, rubbing, water in the connector and loose terminals.
• Check the heater and exhaust sealing, then record the signal transition under the required conditions, including deceleration if specified by the OEM.
• Compare reaction time with the dedicated downstream test; do not expect rapid upstream switching, and check applicable service bulletins.
• After correcting the confirmed fault, repeat the OEM monitor conditions and check for code recurrence, beyond merely extinguishing the lamp.

Also written as: P0159-00, P015900, PO159 (some scan tools add the fault-type byte, e.g. -00).