P0157
O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)P0157 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0157 means
P0157 is stored when the PCM detects that the oxygen sensor signal on Bank 2, Sensor 2 has remained below approximately 0.1 V for longer than the allowed calibration window. Bank 2 is the engine bank that does not contain cylinder 1 — on a V6 or V8 this is typically the opposite bank from Bank 1. Sensor 2 is the downstream sensor located after the catalytic converter on that bank, used to monitor converter efficiency rather than drive active fuel trim. A persistent low-voltage reading indicates the sensor is reporting an excessively lean exhaust condition, or that the signal circuit has an open or short-to-ground fault.
The most common real-world causes are a failed O2 sensor, a broken or corroded signal wire, or an exhaust leak upstream of the sensor diluting the exhaust with ambient oxygen. Because the PCM does not use the Bank 2 Sensor 2 reading to calculate short-term or long-term fuel trim in typical configurations, significant driveability symptoms are uncommon — but the MIL will illuminate and catalyst efficiency monitoring will be compromised. On some vehicles this sensor also contributes to fuel trim in certain drive modes, in which case idle quality and fuel economy may be more noticeably affected.
Common causes
Most-frequently reported root causes when P0157 is logged.
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1
Failed O2 sensor element unable to generate sufficient voltage output.
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2
Open circuit or broken wire in the sensor signal path between the sensor connector and PCM.
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3
Signal wire shorted to ground, clamping output voltage near 0 V.
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4
Corroded or loose connector pins creating high resistance that drops the signal voltage.
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5
Exhaust leak upstream of the sensor introducing ambient oxygen and holding the reading artificially low.
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6
Lean running condition on Bank 2 (e.g. vacuum leak, fuel injector issue) causing genuine low-voltage output.
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7
Failed sensor heater element causing the sensor to remain too cold for accurate output.
Symptoms drivers notice
How to diagnose P0157
A typical diagnostic flow when this code is present.
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1
Read all stored DTCs with a scan tool and record freeze-frame data — note any lean fuel trim codes (P0174) or heater circuit codes that may accompany P0157.
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2
With the engine fully warmed up, observe Bank 2 Sensor 2 live voltage; a healthy sensor should swing 0.1–0.9 V, with a reading stuck below 0.1 V confirming the fault.
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3
Inspect the sensor wiring harness on Bank 2 from the sensor connector back toward the PCM for chafing, heat damage, and corrosion — the Bank 2 sensor wiring run is often longer and more exposed on V-configuration engines.
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4
Check for exhaust leaks at manifold joints, flex pipes, or catalytic converter seams upstream of the sensor on Bank 2.
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5
Backprobe the signal wire at the connector with the engine running; if signal is normal at the connector but absent at the PCM, the wiring between the connector and PCM is at fault.
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6
Replace the oxygen sensor with an OEM-specification Bank 2 Sensor 2 unit if wiring checks pass, and retest live data with the engine warm.
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7
If fuel trim codes are also present on Bank 2, address the lean condition (vacuum leak, injector fault) before or alongside the sensor replacement.
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
Which side of the engine is Bank 2?
Bank 2 is the cylinder bank that does not contain cylinder 1. On most inline-4 engines there is only one bank, so P0157 does not apply. On V6, V8, and some flat engines, Bank 2 is typically the passenger side in front-wheel-drive transverse layouts and the driver's side on many rear-wheel-drive longitudinal V8s — but this varies by manufacturer and model, so always verify with a model-specific reference.
Can P0157 and P0174 (Bank 2 lean) appear at the same time?
Yes. A genuine lean condition on Bank 2 — from a vacuum leak, weak fuel injector, or low fuel pressure — can drive both the upstream fuel trim code P0174 and the downstream low-voltage code P0157 simultaneously. Diagnose and repair any lean condition first, then recheck whether P0157 clears.
Will P0157 put my car in limp mode?
Generally not. Bank 2 Sensor 2 is a downstream monitoring sensor and most calibrations do not use it to enforce limp mode. However, if accompanied by other sensor or fuel system faults, overall system behaviour may be affected.
How do I know if the sensor is bad versus just a wiring fault?
Backprobe the signal wire at the sensor connector with the engine warm. If the sensor generates a proper 0.1–0.9 V swing at the connector, the fault is in the wiring or PCM. If the signal is flat at the connector too, the sensor element is most likely the culprit. Also check the heater resistance across the heater pins — it should typically read 5–20 ohms depending on specification; an open circuit confirms heater failure.
Disabling P0157 in software
RaceTune can permanently disable P0157 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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