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

The downstream (sensor 2) oxygen sensor sits behind the catalytic converter, where its main job is to monitor catalyst efficiency rather than to trim fuel moment-to-moment. Behind a healthy converter its voltage should be relatively steady in the middle of the range. P0157 sets when the bank-2 downstream sensor's voltage stays abnormally low — pinned toward the lean end and not recovering. A low voltage corresponds to a lean or oxygen-rich reading, so the PCM is seeing either real extra oxygen in the exhaust at that point or a sensor/circuit that's reporting low when it shouldn't. 'Bank 2' is the cylinder bank without cylinder number one, and 'sensor 2' is the post-catalyst position.

The causes split between real exhaust conditions and sensor-circuit faults. An exhaust leak near the rear sensor is a classic cause: outside air drawn in reads as extra oxygen and holds the voltage low. A genuinely lean condition — a vacuum leak, low fuel pressure, or a lean-running bank 2 — can do the same, and is often accompanied by lean fuel-trim codes like P0171. On the circuit side, a failed sensor stuck low, a short to ground in the signal wire, a corroded connector, or a heater problem that keeps the element cold can all produce a persistent low reading. Because the downstream sensor isn't the primary fuel-control input, a low reading here is less about immediate driveability and more about whether the catalyst monitor can trust this sensor.

For the driver, P0157 usually means a check engine light with little or no change in how the car runs, though if a real lean condition is behind it you may notice a rough idle or hesitation. It will fail an emissions test and can interfere with catalyst-efficiency monitoring. Diagnosis starts by checking the bank-2 downstream sensor's live data and looking for an exhaust leak near it, then checking fuel trims for a real lean condition, and finally testing the sensor, heater, and wiring before replacing the sensor.