P0137 Code: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
The oxygen sensor after the catalytic converter on bank 1 stays at a low, lean-looking voltage.
At a glance
Severity: Low
- Meaning
- O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $40–$400
- DIY difficulty
- Moderate
Behind a working catalytic converter the oxygen level is low and stable, so the downstream sensor normally reads a fairly steady 0.5 to 0.8 V. P0137 means Bank 1 Sensor 2 stayed below its low threshold, often around 0.2 V or less, for too long, or failed to rise when the PCM ran the mixture rich for a moment to test it. The most common physical reason is outside air: a leaking flange, cracked flex pipe or rusted-through pipe between the converter and the sensor lets oxygen reach the tip, and the sensor faithfully reports lean. Otherwise the sensor has worn out and can no longer generate a rich-side voltage, or the signal wire is grounded where it runs along the floor. A genuinely lean engine or a misfire can pull the rear sensor down too, but then the upstream sensor and fuel trims usually show it as well.
Can I keep driving? Fine to drive in the short term. If misfires or a lean condition are behind it, fix those promptly, because both can damage the catalytic converter.
How to read P0137
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 37 Specific fault
Symptoms
- Check engine light with the car driving normally
- B1S2 sits at 0.05 to 0.2 V even at steady cruise
- Exhaust puffing or ticking noise under the car near the converter
- Catalyst monitor incomplete or failed, sometimes with P0420
- Upstream sensor switching normally while the rear one stays low
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Downstream sensor worn out and unable to produce a rich-side voltage
- Exhaust leak between converter and sensor: flange gasket, cracked flex pipe or rusted pipe
- Signal wire shorted to ground on the underbody, pinched under a clip or crushed by a jack
- Engine running lean overall from a vacuum leak or low fuel delivery
- Misfire sending unburned oxygen through the converter
- Corroded connector leaking the signal to ground
- Aftermarket spacer or incorrect replacement sensor
Diagnosis step by step
Compare the front and rear sensors
With the engine warm, watch B1S1 and B1S2 together along with fuel trims. If the front sensor also reads mostly lean and trims are strongly positive, the whole engine is lean: fix that first. If the front sensor switches normally and trims are near zero while the rear stays low, the problem is behind the converter.
Tools: OBD-II scanner
Push the rear sensor rich
The converter stores oxygen, so a quick throttle blip isn't enough. Accelerate hard for several seconds, for example up a highway on-ramp, or hold propane in the intake at 2,000 rpm for 10 to 20 seconds. A good downstream sensor climbs above 0.8 V once the converter's oxygen is used up; one that stays under 0.3 V is faulty or shorted.
Tools: OBD-II scanner
Find leaks behind the converter
With the exhaust cold, feed smoke into the tailpipe at low pressure or listen with the engine idling for a puffing sound near the flanges. Check the flex joint on front-drive cars carefully, and look for soot streaks around gaskets just ahead of the sensor.
Tools: Smoke machine, Flashlight
Rule out a grounded signal wire
Unplug the sensor, key on, and measure the harness-side signal pin: a bias of about 0.45 V is normal on many systems, while 0 V hints at a short to ground. Inspect where the harness passes clips and the floor pan, especially if the car was recently lifted or had exhaust work.
Tools: Multimeter, Flashlight
Check for misfires
Look at the misfire counters in Mode $06 or live data. Even occasional misfires on one cylinder push oxygen past the converter and drag the rear sensor down, and they must be fixed before a new sensor or converter will stay healthy.
Tools: OBD-II scanner
Replace the sensor and clean the threads
Soak the old sensor with penetrating oil, remove it with an O2 sensor socket and chase the bung with an M18 x 1.5 thread chaser. Install the new sensor at about 30 lb-ft with anti-seize only on the threads, then recheck the rear voltage on a test drive.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace exhaust flange gasket | No | $60–$200 |
| Replace cracked flex pipe section | No | $150–$400 |
| Replace downstream O2 sensor yourself (part) | Yes | $40–$130 |
| Replace downstream O2 sensor at a shop | No | $120–$280 |
| Repair grounded or crushed sensor wire | No | $80–$220 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Field data
| Chevrolet | 3 | Equinox, Malibu, Volt | |
|---|---|---|---|
| Kia | 2 | Sorento |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
Most OBD-II smog programs fail a vehicle with P0137 active. A leak in the exhaust can also be flagged separately in states with a visual or tailpipe inspection, so fix both before retesting.
Frequently asked questions
Does P0137 mean my catalytic converter is bad?
No. A worn-out converter usually makes the rear sensor switch like the front one, which sets P0420. A rear sensor stuck low points to a leak, the sensor itself, its wiring or a lean or misfiring engine.
Why doesn't the rear sensor react when I blip the throttle?
A healthy converter stores oxygen and releases it when the mixture goes rich, so the rear sensor lags behind. It takes several seconds of rich running to use that store up; only then should the rear voltage rise.
Can an exhaust leak really set a sensor code?
Yes, and it is one of the most common reasons for P0137. The leak draws outside air in between exhaust pulses, the sensor sees oxygen and reports lean even though the engine is fine.
Will a new downstream sensor fix it for good?
Only if the old sensor failed the rich test. If it climbed above 0.8 V when you forced the mixture rich, a new one will read low for the same reason; find the leak or misfire first.