P0157 Code: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)

What the P0157 code means

The bank 2 sensor 2 oxygen sensor is the O2 sensor on bank 2 (the side of the engine opposite cylinder 1, which only exists on V-type and horizontally opposed engines), downstream, behind the catalytic converter (sensor 2). The computer uses it to watch catalytic converter efficiency and to fine-tune the correction from the upstream sensor.

With the P0157 code, the sensor signal stays below roughly 0.1 to 0.2 V for too long, as if the mixture were lean all the time. It can be an electrical fault (the signal shorted to ground), an aging sensor, or a real cause: an exhaust leak ahead of the sensor lets in outside air, and the sensor sees extra oxygen.

Because this sensor sits behind the converter, a failure usually does not change how the car drives. It does stop the computer from watching the converter, though, and you will not pass an emissions test until it is fixed.

P0157 code symptoms

• Check engine light on.
• Usually no driveability symptoms.
• Oxygen sensor or catalyst monitors stay incomplete, so the vehicle fails an emissions test.
• Other codes for the same sensor or for catalytic converter efficiency may show up too.

Likely causes of the P0157 code

1. Exhaust leak ahead of the sensor from a manifold gasket, a crack or a broken flex pipe.
2. Aged or contaminated sensor.
3. Signal wire shorted to ground.
4. A mixture that really is lean because of vacuum leaks or low fuel pressure (look at the fuel trims).
5. Misfires that push unburned oxygen into the exhaust.

How to diagnose the P0157 code step by step

1. Read the sensor live data with the engine warm. A healthy upstream sensor swings between 0.1 and 0.9 V several times a second. A healthy downstream sensor stays nearly steady between 0.5 and 0.8 V.
2. Look for exhaust leaks between the engine and the sensor. With the engine cold, listen for puffing or ticking, and hold a hand near the joints and gaskets (without touching them).
3. Inspect the wiring near the exhaust and the connector for oil, melted insulation or corroded pins.
4. Response test: snap the throttle open (the reading should go rich, near 0.9 V) and then let off (it should go lean, near 0.1 V). A slow response means an aged sensor.
5. Check the fuel trims. If they are beyond plus or minus 10 percent, the sensor may be warning you about a real mixture problem.
6. Check the heater. A cold sensor responds poorly even when its element is healthy.
7. Check continuity of the signal and ground wires all the way to the computer.

How to fix the P0157 code

• Repair exhaust leaks before you replace the sensor.
• Repair burned wiring and secure it away from the exhaust.
• If the sensor is contaminated, find and fix the source (oil, coolant, rich mixture) before you install a new one.
• Replace the sensor with one of the correct specification (type, number of wires and wire length).
• Use anti-seize on the threads only, never on the tip of the sensor.
• Clear the codes, drive a full cycle and confirm the sensor monitor reaches ready status. If the P0157 code comes back, retest the circuit before you buy another part.

Technical reference data

• Conventional zirconia sensor: 0.1 V (lean) to 0.9 V (rich).
• Bias voltage with no activity: about 0.45 V.
• Typical heater resistance: 2 to 20 ohms.
• Operating temperature: more than 300 °C (about 570 °F).
• Common thread: M18 x 1.5; typical torque 40 to 50 N·m (about 30 to 37 lb-ft).
• These are reference values only. Check the specifications for your engine.

Brand-specific notes

On four-cylinder inline engines there is only bank 1, so bank 2 codes should never show up. If you see the P0157 code on one of those engines, double-check the code and the vehicle.

P0157 on Toyota and Lexus

On Toyota and Lexus the sensor 1 position is usually a wide-band (air-fuel ratio) sensor, and its equivalent codes are P2195 to P2199 and P2238 to P2243.

P0157 on GM, Toyota and Volkswagen

Sensor 3 positions are common on some GM, Toyota and Volkswagen vehicles that have two catalytic converters in series.

Frequently asked questions about P0157