The oxygen sensor behind the catalytic converter should read fairly steady and slightly rich on a healthy engine. If its voltage stays low, the engine computer thinks there is too much oxygen in the exhaust. A P0137 code means the bank 1 downstream oxygen sensor voltage stayed too low for too long.
This guide explains what pulls the rear sensor low, how to test it and which repairs fix the code.
P0137 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 37 | Specific fault | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
What the P0137 Code Means
The SAE J2012 standard defines the P0137 code as O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2). Sensor 2 is the downstream sensor after the catalytic converter on the cylinder 1 side.
With a warm engine and a working converter, the rear sensor usually holds a steady voltage around 0.5 to 0.8 volt. When the powertrain control module (PCM) sees it stay below about 0.2 volt for a set time, it sets the code.
Some PCMs also run an active test. They make the mixture rich on purpose and expect the rear sensor to rise. If it never does, the code sets.
Symptoms of a Low Rear O2 Sensor Signal
The engine usually runs normally, since the rear sensor has a small role in fuel control. Look for these signs of a P0137 code:
- Check engine light on
- Catalyst monitor that will not complete
- Failed emissions test
- Slight change in fuel economy
- Lean codes stored if the engine is truly lean
Common Causes of the P0137 Code
A rear sensor that reads low most often sees outside air or has a wiring fault. These are the usual causes:
- Exhaust leak near the sensor. Rust holes, cracked pipes and loose clamps let air in.
- Signal wire shorted to ground. Damaged insulation under the vehicle pulls the voltage down.
- Failed oxygen sensor. A worn sensor may stick low.
- Heater fault. A sensor that is too cool produces little voltage.
- Lean running engine. A vacuum leak or low fuel pressure can lower the rear reading too.
- Corroded connector. Resistance in the circuit weakens the signal.
How to Diagnose the P0137 Code
You need a scan tool with live data and a multimeter. Drive the vehicle for 10 minutes or more so the converter is fully hot.
Check Fuel Trims First
Look at bank 1 fuel trims. High positive numbers mean the engine is running lean, which could explain the low rear reading. If trims look normal, the problem is more likely the sensor, wiring or an exhaust leak.
Drive the Mixture Rich
With the engine warm and idling, add a small amount of propane near the intake. The rear sensor should rise to about 0.8 volt or higher within a few seconds. If it rises, the sensor works, so look for an exhaust leak or lean condition. If it stays low, the sensor or its wiring has failed.
Look for Exhaust Leaks and Wiring Damage
With the engine cold, inspect the exhaust from the manifold back past the rear sensor. Look for rust holes, black soot and loose joints. Check the sensor wiring for spots where it rubs on the body or touches the exhaust.
Repairs That Fix the P0137 Code
Repair the cause, clear the code and drive until the monitors complete.
- Fix exhaust leaks near the sensor, such as a cracked flex pipe or loose flange.
- Repair a shorted signal wire.
- Fix vacuum leaks or fuel delivery problems if fuel trims show the engine is lean.
- Replace the downstream oxygen sensor if it fails the propane test.
- Repair the heater circuit if heater codes appear too.
Typical Repair Costs
Costs to fix a P0137 code vary with the cause. This table lists common ranges.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Downstream O2 sensor replacement | $30 to $120 | $120 to $350 |
| Exhaust pipe or flex pipe repair | $30 to $150 | $150 to $500 |
| Wiring repair | $10 to $30 | $100 to $300 |
| Vacuum leak repair | $10 to $80 | $100 to $400 |
Can You Keep Driving with a P0137 Code?
Most of the time, yes. The front sensor still controls fuel, so the engine runs well. However, the PCM cannot properly check the catalytic converter.
An exhaust leak near the sensor can also let fumes into the cabin, especially if the leak is under the passenger area. If you smell exhaust inside, get it fixed right away.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0134 | O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1) |
| P0135 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) |
| P0136 | O2 Sensor Circuit Malfunction (Bank 1 Sensor 2) |
| P0138 | O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) |
| P0139 | O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) |
Fixing a Low Downstream O2 Sensor Signal
A P0137 code means the bank 1 rear oxygen sensor reads too lean for too long. Exhaust leaks, wiring damage and worn sensors cause most cases.
- Check fuel trims for a real lean condition.
- Do a propane test on the rear sensor.
- Look for exhaust leaks near the sensor.
- Replace the sensor only if it fails to rise.
Frequently Asked Questions
Can an Exhaust Leak Cause the P0137 Code?
Yes, and it is one of the most common causes. A leak near the rear sensor lets outside air into the exhaust stream. The sensor reads that oxygen as a lean condition. Rusted pipes and cracked flex joints are frequent culprits on older vehicles.
What Voltage Should the Rear O2 Sensor Read?
On a warm engine with a working catalytic converter, the rear sensor usually holds a fairly steady reading between about 0.5 and 0.8 volt. It should not switch as fast as the front sensor. A reading that stays near 0.1 volt points to a fault or air leak.
Is This Code a Sign of a Bad Catalytic Converter?
Not usually. A failing converter tends to set an efficiency code like P0420, and its rear signal starts switching like the front one. A constantly low reading points more to the sensor, the wiring or an exhaust leak. Fix those first.
Will a Cheap Sensor Fix the Problem?
A low-quality sensor may not match the factory response or heater rating, which can bring the code back. Choose a direct-fit sensor from a known brand for your exact engine and position. It costs a little more but saves repeat repairs.