OBD-II trouble code
P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor on bank 1 is reporting a voltage that's stuck low — sitting near zero and refusing to rise. The usual culprits are a signal wire shorted to ground, an open circuit, an exhaust leak pulling in fresh air, or a worn-out sensor.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0137 mean?
P0137 is the low-voltage member of the downstream-sensor family on bank 1. It's closely related to P0136 (the general circuit code for the same sensor) and is the mirror image of P0138 (stuck high). Where P0136 can be set by a range of downstream faults, P0137 specifically means the PCM is watching the sensor signal sit pinned near 0V — a flat, low reading that a working post-catalyst sensor should never produce.
The downstream oxygen sensor lives behind the catalytic converter. Its purpose is to confirm the catalyst is doing its job, not to control fuel delivery. A conventional zirconia sensor, the type Bosch describes in Fluke's multimeter note, swings from about 0.1 V lean to about 0.9 V rich and pivots near 0.45 V. That swing belongs to that sensor. It is not the resting voltage of every sensor fitted downstream of a catalyst. A reading stuck at the bottom of the scale tells the PCM one of a few things, and the failure modes for a low signal are distinct from the high-signal causes that set P0138.
The electrical fault that produces a pinned-low reading is one that pulls the signal down — a signal wire chafed against the chassis or exhaust and shorted to ground, or a corroded connector leaking to ground. An open circuit is a different result. The module holds an open input at a bias, and that bias is the one in the service information for this vehicle, not the 0.45 V zirconia pivot. A broken wire or a backed-out pin sits at that bias. A reading genuinely pinned at 0 V points at ground, not at a break. The second common cause is an exhaust leak ahead of the sensor — a cracked downpipe, a leaking flange, or a pinhole in the catalyst housing lets atmospheric air reach the sensor, and all that extra oxygen makes the sensor read lean (low voltage) even though the engine isn't actually lean. The third is simple sensor wear; an aged element loses its ability to generate voltage and drifts low. That gives you a field test with a clear verdict either way. Unplug the sensor and watch the PCM's reading: if it rises to the bias voltage, the harness is intact and the sensor itself was dragging the line down; if it stays pinned at 0V with the sensor out of the circuit, the signal wire is shorted to ground.
Common causes
- Signal wire shorted to ground — chafed insulation contacting the chassis or exhaust
- Corroded or leaking connector providing a path to ground on the signal pin
- Exhaust leak ahead of the downstream sensor, pulling in atmospheric air and skewing the reading lean
- Corroded or water-intruded sensor connector
- Worn or aged downstream O2 sensor that no longer generates proper voltage
- Failed sensor heater (often sets P0141 alongside P0137), keeping the sensor too cold to read correctly
- Damage to the harness from aftermarket exhaust work
- Catalytic converter damage altering the exhaust the sensor sees
Symptoms
- Check Engine Light on, usually with normal drivability
- Failed emissions test or a catalyst monitor that won't reach 'ready'
- Faint exhaust smell on cold start if a leak ahead of the sensor is the cause
- Minor loss of fuel economy
- No noticeable power or idle change in most cases
- Occasionally a companion heater-circuit code (P0141)
Diagnostic steps
- 1.Read live data for bank-1 sensor-2 voltage. A value pinned near 0V that won't move confirms a genuine stuck-low signal rather than a glitch.
- 2.Unplug the sensor and watch the PCM reading. A rise to the input's bias voltage — the bias in the service information, not the 0.45 V zirconia pivot Bosch describes in the Fluke note — means the harness can go high and the sensor was pulling it down. A reading still pinned at 0 V with the sensor disconnected means the signal wire is shorted to ground.
- 3.Inspect the signal wire end to end, especially where it runs near the chassis and exhaust, for chafe points shorting to ground.
- 4.Check the connector for corrosion, moisture, and backed-out pins. A poor connection often reads as an open or a low signal.
- 5.Listen for an exhaust leak ahead of the sensor — a small leak introduces air that pulls the signal low without any real lean condition. Listen on a cold start rather than feeling along a hot pipe; a leak you can hear is a leak you don't need to burn a hand finding.
- 6.Confirm the sensor heater is working — check for supply voltage at the heater feed with the key on, bearing in mind that some platforms only energise it with the engine running. A cold zirconia sensor generates no usable output at all, so a dead heater leaves the PCM with nothing to read; whether that logs as stuck-low or as no activity depends on the platform.
- 7.Before any of this, work safely: the downstream sensor and the pipe it threads into run at several hundred degrees, so let the exhaust cool before touching it, never feel along the pipe for a leak with the engine running, and if the car has to come off the ground put it on level ground with proper stands rather than a jack alone.
- 8.If wiring, connector, and exhaust all check out, the sensor is what's left to test. Don't plan on swapping it with the bank-2 sensor to prove it. On a V-engine the swap means breaking loose and reinstalling two sensors in old exhaust threads, which is how a sensor swap becomes a broken bung. Judge it from the live data and the circuit tests instead.
Repair cost
$50 – $600
A single chafed wire is the inexpensive repair. A downstream sensor, or an exhaust leak buried where the pipe is awkward, is the expensive one. Read the bracket as a planning span rather than a quoted price. Price the wire repair or the sensor, then add labor as the service information states it for the car in front of you. Rule out the wiring and the exhaust leak before a sensor goes on.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.