P0137: The downstream oxygen sensor on the driver side is reading too low
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
Yes, you can drive for now. The downstream sensor does not control fuel delivery directly. Fix it within a week or two, since a stuck-low reading can eventually set a false catalyst code.
What it means
The downstream oxygen sensor on Bank 1 should oscillate slowly between about 0.1 and 0.9 volts. P0137 means it stayed below 0.1 volts longer than the computer expected. A healthy converter produces a fairly steady low-oxygen signal downstream. A reading stuck at low voltage usually means the sensor is responding only to lean exhaust, has contaminated its sensing element, or has a wiring problem pulling its signal to ground. The non-obvious point: a sensor stuck low can fool the computer into thinking the catalytic converter is working perfectly when it may not be.
Common causes
- Worn or contaminated downstream O2 sensorOil ash, coolant, or just age degrades the sensing element. The sensor gets stuck near zero.
- Lean exhaust condition at the converter outletAn exhaust leak upstream of the sensor pulls in air and keeps the sensor reading low.
- Short to ground in the signal wireDamaged insulation on the signal wire can pin the voltage low regardless of sensor condition.
- Open in the sensor reference voltage wireIf the PCM reference voltage is not reaching the sensor, output stays at zero.
What it costs
- Wiring or connector repair$80 to $200
- Downstream O2 sensor (Bank 1)$130 to $300 installed
Symptoms
- Check engine light on, steady
- Usually no drivability symptoms
- Possibly missing a related P0420 that would otherwise be set
- Failed emissions test
What to do next
Check for exhaust leaks between the upstream sensor and the downstream sensor first. A leak is easy to diagnose and cheap to fix. If no leak, inspect the connector and harness. If both check out, replace the sensor. Do not assume P0420 is absent because this code is set; a stuck-low sensor can suppress that code.
Lewis charts the downstream sensor voltage trace over time so you can see whether it is flatlined or occasionally oscillating. Download Lewis.
Related codes
FAQ
- Is P0137 the same as P0136?
- No. P0136 is a general circuit fault. P0137 is a specific voltage-too-low fault. P0137 often points to the sensor element or a wiring short to ground, while P0136 can also mean an open circuit.
- Can a catalytic converter cause P0137?
- A very efficient, new converter keeps downstream oxygen low, but not low enough to trigger this code under normal conditions. If anything, P0137 can mask a failing converter.
- Will cleaning the sensor fix it?
- Rarely. Oxygen sensors are not serviceable. If the element is contaminated, the sensor needs replacement.