P0159
O2 Sensor Circuit Slow Response Bank 2 Sensor 2.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 59
- FaultThe specific fault in that area
What it means
P0159 means the control unit has judged the post-catalyst oxygen sensor on bank 2 (the side of the engine opposite cylinder 1) to be responding too slowly. During its monitoring routine the signal switched too lazily, or did not move as far or as fast as expected compared with the upstream sensor. The exact test conditions and thresholds vary by model and engine.
Symptoms
- Engine warning light on
- Usually no noticeable change in driving
- Possible slight rise in fuel consumption
- Emissions or MOT test failure
- Occasionally a rough idle or hesitation if a mixture fault is also present
Possible causes
- Ageing, contaminated or failing bank 2 downstream oxygen sensorlikely
- Exhaust leak near the bank 2 downstream sensor, or at a joint upstream of it, letting in airlikely
- Damaged wiring, poor connector contact or corrosion in the sensor circuitpossible
- Sensor contaminated by oil, coolant, silicone or fuel additive residuepossible
- Worn catalytic converter on bank 2 altering the downstream signalpossible
- Persistent rich or lean mixture fault on bank 2, such as a fuel delivery or air leak problempossible
- Control unit fault or outdated calibration (model specific, check manufacturer bulletins)rare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting engine temperature, load and speed when the fault set.
- Check for other codes, especially catalyst efficiency, other oxygen sensor, misfire or fuel trim codes, and diagnose those first as they may be the root cause.
- Visually inspect the bank 2 downstream sensor wiring and connector for chafing, heat damage, water ingress and corrosion.
- Inspect the exhaust from the manifold to beyond the sensor for leaks, cracks, loose joints or blowing gaskets.
- Check short and long term fuel trims on bank 2 at idle and at steady 2500 rpm to spot mixture problems.
- Watch the live downstream sensor signal on a warm engine and compare it with the upstream sensor. A healthy signal is relatively steady, and it should respond briskly to a deliberate rich or lean change.
- Check the sensor heater circuit and the supply and ground, using the manufacturer's wiring data for your model.
- Look for signs of contamination on the sensor tip, and check for oil or coolant consumption if it is heavily fouled.
How to fix
- Repair or replace damaged wiring and clean or renew the faulty connector.
- Repair the exhaust leak and renew any failed gaskets or sections.
- Replace the bank 2 downstream oxygen sensor with a correct specification part if it is confirmed slow or contaminated.
- Correct the underlying cause of contamination, such as an oil or coolant leak, before fitting a new sensor.
- Fix any mixture fault found, such as a vacuum leak or fuel delivery problem.
- Replace the catalytic converter only if testing confirms it has failed, and update the control unit software only if the manufacturer lists it as a fix for your model.
Can I keep driving?
Low severity
The car can usually be driven normally while you book it in for diagnosis. Fix it promptly, because an underlying mixture or exhaust problem could affect fuel economy, the catalytic converter and emissions test results.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.