P0157 — Post-Cat O2 Sensor Low Voltage (Bank 2 Downstream)

What does P0157 mean?

P0157 is the Bank 2 counterpart of P0137. The downstream oxygen sensor on the cylinder bank that does NOT contain cylinder number 1 is reporting a voltage below the calibrated minimum — typically under about 0.1 V — for an extended period. Sensor 2 sits after the catalytic converter, so on a warm engine it should hold a relatively steady 0.6–0.8 V rather than switching. A reading pinned low points at an open or shorted-to-ground signal circuit, a failed sensor, fresh air entering the exhaust ahead of the sensor, or a genuine lean condition on that bank.

What are the symptoms of P0157?

• Check Engine Light is illuminated
• Catalyst readiness monitor will not complete
• Often no drivability symptoms at all
• Possible slight reduction in fuel economy
• Vehicle will fail emissions / smog testing
• Sometimes accompanied by P0174 if Bank 2 is genuinely lean

What causes P0157?

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0157?

In most cases a vehicle stays drivable for short trips with P0157 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0157

1. Identify Bank 2, then identify Sensor 2

   Two chances to go to the wrong part here. Bank 2 is the bank without cylinder number 1 — confirm which physical side that is in service information, because it varies by manufacturer. Sensor 2 is the sensor AFTER the catalytic converter on that bank, not the one in the manifold. Get either wrong and you replace a healthy sensor.

   Tools: Vehicle-specific service information, Vehicle lift or jack stands

2. Compare Bank 2 and Bank 1 fuel trims

   This is the fastest way to tell a lying sensor from a real problem. If Bank 2 long-term fuel trim is near zero, the engine is fuelling correctly on that bank and the sensor or its wiring is at fault. If Bank 2 trim is high positive while Bank 1 is normal, you have a genuine bank-specific lean condition — find the leak rather than replacing the sensor.

   Tools: Scan tool with per-bank fuel trim PIDs

3. Disconnect the sensor and check for bias voltage

   Unplug the Bank 2 downstream sensor and turn the key on. The scan tool PID should rise to the ECM's bias voltage, typically around 0.4–0.5 V. If it stays pinned near zero with the sensor disconnected, the signal wire is shorted to ground between the connector and the PCM, and no sensor will fix that.

   Tools: Scan tool, Multimeter

4. Inspect the harness under the vehicle

   Follow the wiring from the sensor to the chassis connector. Look for insulation melted by exhaust contact, chafe points against the underbody, and corrosion or water inside the connector. On many V engines the Bank 2 downstream harness takes a longer route across the underside of the car, giving it more places to fail.

   Tools: Flashlight, Inspection mirror, Electrical contact cleaner

5. Look for exhaust leaks between the Bank 2 catalyst and sensor

   A pinhole, a cracked weld, or a leaking flange gasket ahead of the sensor lets fresh air into the stream, and the sensor honestly reports the oxygen it finds — as a permanently low voltage. Listen along the pipe with the engine cold, when leaks are loudest before the metal expands.

   Tools: Mechanic's stethoscope, Smoke machine (optional), Flashlight

How do I fix P0157?

• Replace the Bank 2 downstream O2 sensor with an OEM part
• Repair the shorted-to-ground or open O2 signal wire
• Repair exhaust leaks between the Bank 2 catalyst and its downstream sensor
• Clean or replace the corroded sensor connector
• Repair Bank 2 vacuum leaks if fuel trims confirm a real lean condition

P0157 by make and model

Select your exact year, make, and model below to get a P0157 diagnosis tuned to your vehicle's known failure patterns, recalls, and TSBs.

Related diagnostic codes

• P0137 — Post-Cat O2 Sensor Low Voltage (Bank 1 Downstream)
• P0156 — Post-Cat O2 Sensor Circuit Fault (Bank 2)
• P0174 — Fuel System Too Lean (Bank 2)
• P0420 — Catalyst Efficiency Below Threshold
• P0300 — Random or Multiple Cylinder Misfire
• P0171 — Fuel System Too Lean (Bank 1)

Common symptoms of P0157

P0157 often shows up as one of these symptoms. Start from what your car is actually doing:

• Poor fuel economy
• Failed emissions / smog test