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P0157 — O2 Sensor Circuit Low Voltage Bank 2 Sensor 2

Generic OBD-II · all makesPowertrain (engine & transmission)

P0157 indicates that the powertrain control module has detected a low voltage signal from the downstream (post‑catalyst) oxygen sensor on bank 2, sensor 2. This usually means the sensor is reporting voltage below the expected minimum range.

Symptoms

What causes it — most likely first

1. Faulty bank 2 sensor 2 (downstream O2 sensor)

The sensor's heater or sensing element may have failed, causing voltage to stay low.

2. Wiring harness damage or poor connector contact

Corroded pins, broken wires, or loose connectors can drop voltage to the sensor.

3. Excessive exhaust leak upstream of sensor 2

A leak before the sensor can introduce extra oxygen, pulling the sensor voltage down.

4. Failed sensor heater circuit

If the heater does not reach operating temperature, the sensor may not generate proper voltage.

5. Powertrain control module (PCM) fault

Rare; only considered after ruling out sensor, wiring, and exhaust issues.

How it's diagnosed

  1. Inspect sensor 2 wiring and connectors for corrosion, damage, or looseness; repair as needed.
  2. Measure sensor voltage with a scan tool while the engine is at operating temperature; compare to expected range (typically >0.1 V).
  3. Test sensor resistance and heater circuit continuity with a multimeter.
  4. Check for exhaust leaks upstream of sensor 2 using visual inspection or smoke test.
  5. If all hardware tests are normal, perform a PCM re‑flash or swap test to confirm module integrity.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationSAE generic — identical on every make
Related codesP0150 — O2 Sensor Circuit Bank 2 Sensor 1P0151 — O2 Sensor Circuit Low Voltage Bank 2 SP0152 — O2 Sensor Circuit High Voltage Bank 2 P0153 — O2 Sensor Circuit Slow Response Bank 2P0154 — O2 Sensor Circuit No Activity DetectedP0155 — O2 Sensor Heater Circuit Bank 2 Sensor

Frequently asked

Do I need to replace the PCM for a P0157 code?

In most cases no. The PCM is rarely the cause; sensor, wiring, or exhaust issues are far more common. Replace the PCM only after exhaustive verification that all other components are functional.

Can a bad battery or charging system trigger P0157?

A severely low system voltage can affect sensor readings, but a healthy charging system is usually not the primary cause of a low‑voltage O2 sensor code.

Will fixing P0157 improve fuel economy?

Yes, restoring proper O2 sensor feedback allows the engine to maintain optimal air‑fuel ratios, which can improve mileage and reduce emissions.

Is it safe to drive with this code active?

The vehicle will generally run, but you may experience reduced performance, higher emissions, and possible fuel‑trim issues. It’s best to diagnose and repair promptly.