P0157

What it means

P0157 means the engine control unit has seen a persistently low voltage from the downstream oxygen sensor on bank 2, sensor 2, which sits after the catalytic converter. The signal has stayed below the expected range, often close to zero volts or stuck lean, instead of switching around its normal mid-range. This points to a sensor, wiring or exhaust leak problem rather than a fault with the catalyst itself.

Symptoms

• Engine warning light on
• Usually no noticeable change in driving
• Possible slight rise in fuel consumption
• Failed emissions test
• Occasional rough running if a fuelling fault is also present

Possible causes

• Failed or aged downstream oxygen sensor on bank 2 (sensor 2)likely
• Damaged, chafed or melted sensor wiring, or a short to earth in the signal circuitlikely
• Corroded or loose connector at the sensor or control unitpossible
• Exhaust leak near the sensor letting in extra air and skewing the readingpossible
• Sensor heater circuit fault or contamination of the sensor by coolant, oil or siliconepossible
• Genuine fuelling fault on bank 2, such as a lean condition from a vacuum leakrare
• Fault in the engine control unitrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including engine load, temperature and fuel trims
2. Check for other fuelling, misfire or oxygen sensor codes that might point to a wider problem
3. Confirm which side is bank 2 and which sensor is sensor 2 for this engine, as layouts vary by model
4. Inspect the sensor wiring and connector for heat damage, chafing, water ingress and corrosion
5. Check the exhaust for leaks upstream of and around the sensor
6. View live data for the bank 2 sensor 2 signal and see whether it stays low or responds to throttle changes and a brief enrichment
7. Check the sensor circuit for supply, earth and signal wiring continuity and shorts against the manufacturer's wiring data
8. Compare fuel trims on bank 2 to rule out a real lean condition before condemning the sensor

How to fix

• Repair or replace damaged wiring and clean or replace faulty connectors
• Replace the bank 2 downstream oxygen sensor if it is slow, stuck low or has failed its tests
• Repair any exhaust leak near the sensor and renew the gasket or section as needed
• Correct any underlying fuelling fault, such as a vacuum leak, and trace the source of any contamination
• Clear the code after repair and road test to confirm it does not return

Can I keep driving?

Nearby codes

• P0156GenericO2 Sensor Circuit Bank 2 Sensor 2
• P0158GenericO2 Sensor Circuit High Voltage Bank 2 Sensor 2