P0157: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
The downstream oxygen sensor on bank 2, after the catalytic converter, is reading low voltage for too long.
What P0157 means
P0157 means the bank 2 downstream oxygen sensor has stayed below its low voltage limit, usually around 400 millivolts, for longer than allowed. This sensor sits after the bank 2 catalytic converter and is mainly used to judge converter health, though some cars also use it to fine tune fuel. A constant low voltage tells the computer there is a lot of oxygen after the converter. That can be real, from an exhaust leak letting in outside air or a lean engine, or it can be a false reading from a failed sensor or a signal wire shorted to ground. Exhaust leaks are especially common here because the sensor sits near flanges and pipes that rust. Shops check the upstream sensor and fuel trims first. Then they look for leaks and test the downstream sensor by forcing the engine rich; a good sensor should climb above about 0.8 volts. If it stays low, the sensor or its wiring is replaced.
Common symptoms
- Check engine light on
- Usually no change in how the car drives
- Failed emissions test
- May appear with P0430
- Exhaust smell in some cases
Most common causes
- Failing bank 2 downstream oxygen sensor
- Exhaust leak ahead of the sensor
- Signal wire shorted to ground
- Heater failure keeping the sensor cold
- Lean running engine on bank 2
What to check yourself first
Look under the car for rusted pipes, loose clamps or a leaking flange near the bank 2 downstream sensor. Inspect the wiring and connector for road damage and corrosion.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | When the sensor will not rise even with the engine running rich. |
| Exhaust leak test | $61-$90 | Finds exhaust leaks near the sensor. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For shorted or damaged wiring under the car. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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