P0137: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
The oxygen sensor after the catalytic converter on bank 1 is reading low voltage for too long, which looks like lean exhaust.
What P0137 means
RepairPal explains that P0137 sets when the downstream oxygen sensor voltage stays below about 400 millivolts for more than roughly 20 seconds, with exact values varying by make and model. Since a converter normally keeps the downstream reading somewhat rich, a constant low reading suggests too much oxygen after the converter. That can come from an exhaust leak letting in outside air, a truly lean engine, or a sensor that has failed or whose wiring is shorted to ground. RepairPal lists a broad set of causes: a bad sensor or heater, exhaust leaks, a failing converter, intake leaks, low fuel pressure, a bad coolant temperature sensor, wiring faults, or software that needs an update. Shops start by checking fuel trims and the upstream sensor. If the engine mixture is fine, they look for exhaust leaks near the downstream sensor and test the sensor by adding fuel to see whether its voltage rises. A sensor that will not rise is replaced.
Common symptoms
- Check engine light on
- Possible rough idle
- Lower fuel economy
- Bad smelling exhaust
- Sometimes no symptoms
Most common causes
- Defective downstream oxygen sensor
- Exhaust leak ahead of the sensor
- Failed sensor heater
- Vacuum leak or low fuel pressure
- Faulty engine coolant temperature sensor
- Wiring short to ground
What to check yourself first
Check for exhaust leaks around the catalytic converter flanges and pipes, especially rust holes. Look at the downstream sensor wiring. Check fuel trims; if they are high, the engine may really be lean.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | When the sensor fails a response test. |
| Smoke test for exhaust or vacuum leaks | $61-$90 | Finds exhaust or intake leaks that can cause low readings. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a shorted wire or heater circuit issue. |
| Replace engine coolant temperature sensor | $499-$585 | RepairPal lists a bad coolant temperature sensor as a possible cause. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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