What Does P0137 Mean?
The ECM has detected that the downstream oxygen sensor (bank 1 sensor 2) voltage is stuck low, consistently reading below the expected minimum threshold. For a downstream sensor monitoring a healthy catalytic converter, the voltage should be relatively stable. A persistently low reading could indicate a lean exhaust condition reaching the sensor, a sensor bias fault, or a wiring short to ground.
Common Causes
35%
Failed downstream O2 sensor with lean-biased sensing element or contamination preventing proper voltage generation
25%
Exhaust leak between the catalytic converter and the downstream sensor introducing ambient air and pulling voltage lean
20%
Short to ground in the O2 sensor signal wire from underbody damage, corrosion, or harness chafing on frame rails
20%
Catalytic converter over-performing (excessive oxygen storage) causing genuinely low downstream oxygen levels
Diagnostic Steps
1
Monitor B1S2 O2 voltage during steady driving. If voltage is stuck below 0.1V, disconnect the sensor; if voltage rises at the ECM connector, the sensor is shorted internally.
2
Inspect the exhaust system between the catalytic converter outlet and the sensor bung for leaks. A smoke test of the exhaust system can reveal small leaks that are difficult to find visually.
3
Check the signal wire for damage between the underbody sensor connector and the ECM. Look for chafing on frame rails, heat shield contact points, and subframe mounting areas.
4
Compare B1S2 with B1S1 readings. If the upstream sensor is showing normal cycling but the downstream is stuck low, the fault is isolated to the downstream sensor circuit or a post-catalyst exhaust leak.
5
Verify there are no other DTCs for lean conditions (P0171) that would indicate a systemwide lean condition being correctly reported by the downstream sensor.
Estimated Repair Cost
$75 - $300
Parts + labor, varies by vehicle and location
The ECM has detected that the downstream oxygen sensor (bank 1 sensor 2) voltage is stuck low, consistently reading below the expected minimum threshold. For a downstream sensor monitoring a healthy catalytic converter, the voltage should be relatively stable. A persistently low reading could indica...
The most common cause of P0137 (Oxygen Sensor Low Voltage, Engine Back A, Downstream of Catalytic Converter) is: Failed downstream O2 sensor with lean-biased sensing element or contamination preventing proper voltage generation
Typical repair costs for P0137 range from $75 to $300, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A low downstream O2 reading primarily affects the catalyst efficiency monitor and may cause an incorrect catalyst efficiency DTC. It has minimal impact on engine operation or safety but will cause emissions test failure.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0137 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Emissions Monitoring System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
