What it means
The ECU detects abnormally low voltage from the downstream O2 sensor on bank 2. Indicates persistently lean exhaust past the catalyst, open sensor or wiring shorted to ground.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 2 sensor 2 is the oxygen sensor downstream of the catalytic converter on bank 2. P0157 sets when its signal voltage stays lower than the ECU’s expected range; this can result from a genuinely oxygen-rich exhaust reading or a sensor signal circuit pulled low.
Common causes
Most likely first.
- Lean condition on bank 2
- Signal wire shorted to ground
- Downstream O2 sensor failed
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Capture freeze-frame data and check live data for bank 2 sensor 2 voltage alongside bank 1 sensor 2 to confirm whether the low reading persists when warm.
- Inspect the downstream bank 2 sensor wiring and connector for a signal wire short to ground, damaged insulation, corrosion, or loose terminals.
- Check for exhaust leaks near bank 2 sensor 2 and compare bank 2 fuel trims and upstream sensor data for evidence of a real lean condition.
- Test the signal wire for shorts and continuity, and check sensor power and ground as applicable using the wiring diagram and manufacturer’s specification.
- If wiring is intact and the exhaust or mixture does not explain the reading, test bank 2 sensor 2 and replace it only if it fails.
Common fixes
- Repair a signal wire short to ground or damaged connector.
- Fix an exhaust leak near the downstream bank 2 sensor.
- Correct a confirmed lean condition affecting bank 2.
- Replace bank 2 sensor 2 if it produces an abnormally low signal despite a sound circuit.
Can I keep driving?
A steady light usually allows a careful short drive, but the ECU may not reliably monitor the bank 2 catalyst and emissions can rise. Stop if the engine runs severely rough or the warning light flashes.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.