P0137 – O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

What does P0137 mean?

The downstream oxygen sensor (Bank 1, Sensor 2) is reporting persistently low voltage, indicating a lean exhaust condition or a sensor that has stopped responding. This sensor sits after the catalytic converter and monitors converter efficiency rather than controlling fuel delivery.

What causes P0137?

Listed from most to least common:

1. 1Aged or failed downstream oxygen sensor
2. 2Exhaust leak near or ahead of the sensor drawing in outside air
3. 3Damaged wiring or corroded connector
4. 4Genuine lean condition from a vacuum leak or fuel delivery problem
5. 5Sensor contamination from coolant or oil consumption

Code breakdown

Common symptoms

• Check engine light on
• Usually normal drivability
• Slightly reduced fuel economy
• Failed emissions test

How to diagnose

1. 1Check for other codes first — if lean codes such as P0171 are also present, fix those first, since the sensor may be reporting accurately.
2. 2Inspect the exhaust for leaks between the converter and the sensor.
3. 3Graph downstream O2 voltage. It should sit relatively steady around 0.6–0.7V once warm, not pinned low.
4. 4Inspect the connector and wiring for road damage and corrosion, since this sensor is underneath the vehicle.
5. 5Consider sensor age — replacement is common past 100,000 miles.

Drivability is usually normal, but emissions monitors will not complete and the vehicle will fail inspection.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

Do not replace this sensor before ruling out an exhaust leak. A small leak ahead of the sensor introduces oxygen and produces exactly this reading with a perfectly healthy sensor.

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