P0156 Code: O2 Sensor Circuit Malfunction (Bank 2 Sensor 2)

What the P0156 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.

The P0156 code means the computer found a general electrical fault in the sensor circuit: a signal that is out of range, does not change, or does not make sense compared with the sensor heater. It is the generic version of the low voltage, high voltage and no activity codes.

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.

P0156 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 P0156 code

1. Damaged or oily sensor connector. This is the most common cause.
2. Burned or melted wire from touching the exhaust.
3. Failed sensor with a damaged element or damaged internal wiring.
4. Poor ground at the sensor or at the computer.
5. Computer failure. Very uncommon.

How to diagnose the P0156 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 P0156 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 P0156 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 P0156 code on one of those engines, double-check the code and the vehicle.

P0156 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.

P0156 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 P0156