P0137
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor signal is fixed at a low voltage. Either the catalyst is letting unburned oxygen through (sensor sees lean), the sensor itself is shorted to ground, or the catalyst is so degraded that the downstream signal mirrors a lean upstream condition.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Common causes
Ordered by how often each turns out to be the cause.
- Signal wire shorted to groundPossible
- Catalyst degraded (often appears before P0420)Possible
- Exhaust leak between catalyst and downstream sensorLess likely
Parts involved
- Oxygen Sensor Downstream
- Catalytic Converter
P0137 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.