Failed downstream oxygen sensor (bank 1 sensor 2) that is stuck at a low voltage; Damaged or corroded wiring between the sensor and the engine computer, especially a short to ground; Exhaust leak upstream of the sensor allowing outside air to skew the reading; Contaminated sensor from coolant, oil, or silicone poisoning; Faulty engine computer driver circuit for the sensor heater or signal return; Low fuel pressure or a leaking injector causing an excessively lean exhaust stream; Aging catalytic converter that no longer stores oxygen properly, though this usually sets a different code

DTC codes / P01xx · Fuel and air metering / P0137

P0137O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

Q: What does code P0137 mean?

O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2). P0137 indicates that the downstream oxygen sensor on bank 1 (often called sensor 2) is reporting a voltage that stays below the normal operating range for too long. The engine computer expects this sensor's signal to oscillate between roughly 0.1 and 0.9 volts once the catalytic converter is hot, and a sustained low voltage tells the computer the sensor circuit is either shorted to ground, the sensor is lazy, or the exhaust stream is unusually oxygen-rich.

P0137 indicates that the downstream oxygen sensor on bank 1 (often called sensor 2) is reporting a voltage that stays below the normal operating range for too long. The engine computer expects this sensor's signal to oscillate between roughly 0.1 and 0.9 volts once the catalytic converter is hot, and a sustained low voltage tells the computer the sensor circuit is either shorted to ground, the sensor is lazy, or the exhaust stream is unusually oxygen-rich.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Slightly rough idle or hesitation during acceleration in some cases
A faint rotten-egg or fuel smell from the exhaust if the mixture is rich
Poor fuel economy as the computer adjusts fuel trim based on faulty feedback
Failed emissions test due to the sensor readiness monitor not completing
In rare cases, a drop in power or surging while cruising

Common causes, most likely first

Failed downstream oxygen sensor (bank 1 sensor 2) that is stuck at a low voltage
Damaged or corroded wiring between the sensor and the engine computer, especially a short to ground
Exhaust leak upstream of the sensor allowing outside air to skew the reading
Contaminated sensor from coolant, oil, or silicone poisoning
Faulty engine computer driver circuit for the sensor heater or signal return
Low fuel pressure or a leaking injector causing an excessively lean exhaust stream
Aging catalytic converter that no longer stores oxygen properly, though this usually sets a different code

How to diagnose and fix it

Connect a scan tool and record freeze-frame data and pending codes to confirm P0137 is active and not intermittent
Inspect the downstream oxygen sensor wiring and connector for chafing, corrosion, or a short to ground
Check for exhaust leaks between the engine and the sensor using a smoke machine or by listening for ticking
Measure the sensor signal voltage with a digital voltmeter or oscilloscope while the engine is running to see if it is pinned low
Test the sensor heater circuit resistance and compare it to the manufacturer's specification
Replace the downstream oxygen sensor if the wiring and exhaust are sound and the signal remains low
Clear codes and complete a drive cycle to verify the repair and confirm the monitor runs without resetting the code

Why a low voltage does not always mean a bad sensor

A common misdiagnosis is replacing the downstream sensor when the real problem is an exhaust leak or a failing upstream sensor that is skewing fuel trim and driving the downstream signal low.
The downstream oxygen sensor on bank 1 sensor 2 is often a monitoring sensor only, so a low voltage may not affect driveability at all and the code can be set by a lazy sensor that still switches but not fast enough.
Wiring shorts to ground are frequently caused by the sensor harness rubbing against the exhaust shield or suspension components, so a visual inspection of the entire run is essential before condemning the sensor.
A contaminated sensor from a previous coolant or oil leak can read low even after the leak is fixed, and the code may return until the sensor is replaced.
On some vehicles, a weak battery or poor ground can cause the reference voltage to drop and set a false low-voltage code, so checking charging system health is a worthwhile first step.

Can I keep driving with P0137?

You can usually continue driving with P0137 because the downstream sensor mainly monitors catalytic converter efficiency and does not control fuel delivery directly. Avoid ignoring it for months, though, since a persistent low voltage can mask a richer underlying problem that may eventually damage the catalytic converter or increase emissions.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.