P0137 Code: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)

What the P0137 code means

The sensor after the catalytic converter does not correct the mixture. Its job is to let the system judge whether the converter is still cleaning the exhaust. With a healthy converter, its signal stays steady at mid-range values, typically between 0.6 and 0.8 volts, without the fast swings of the upstream sensor.

The P0137 code is stored when that signal stays below what is expected for longer than allowed. A low value corresponds to excess oxygen in the exhaust gases.

The most common cause is not in combustion but in the exhaust. A leak between the engine and the sensor lets air in during the low-pressure pulses, and the sensor reads it as leftover oxygen even though the mixture is fine.

The other two possibilities are an open circuit, which produces odd readings, and a worn-out sensor that can no longer generate enough voltage. A third one is less obvious: if the mixture really is lean because of an intake leak or low fuel pressure, the downstream sensor will show that too.

It is worth putting the real priority of this code in perspective. The downstream sensor takes no part in the fuel calculation, so the engine runs exactly the same with it or without it, and no driver would notice the failure from behind the wheel. Its only job is to allow the converter to be evaluated, a check that happens once per drive cycle. That lets you deal with it without urgency, but do not forget it: inspection day arrives, the monitor shows as incomplete and the rejection is automatic even if everything else on the vehicle is perfect.

It also helps to know why this sensor reads high and steady when the converter works. A converter in good shape stores oxygen when the mixture is briefly lean and releases it when the mixture is rich, smoothing out the variations. The result is that the gases reaching the downstream sensor carry very little free oxygen, which shows up as a high, constant voltage.

That is why an unusually low signal points to oxygen that should not be there, and why the first suspect is air coming in through a leak rather than a combustion problem.

P0137 code symptoms

• Check engine light on.
• Engine runs with no noticeable change.
• Puffing or metallic ticking from the exhaust, especially when cold.
• The catalytic converter monitor does not reach ready status.
• Failed emissions inspection.
• Slight increase in fuel use.
• Lean mixture codes may be stored alongside it.
• The signal stays low even when the upstream sensor is keeping the mixture under proper control.

Likely causes of the P0137 code

1. Exhaust leak between the engine and the sensor. The most frequent cause.
2. Blown exhaust manifold gasket.
3. Signal circuit open or with high resistance.
4. Sensor worn out by mileage.
5. Connector with water or corrosion, common because of its spot under the vehicle.
6. Wiring damaged under the vehicle.
7. A mixture that really is lean because of intake leaks or low fuel pressure.
8. Circuit ground with high resistance.
9. Failed sensor heater, which prevents a valid reading.

How to diagnose the P0137 code step by step

1. Start the engine cold and listen to the exhaust. A puffing sound that fades as it warms up confirms a leak, because the metal expands and partly seals it.
2. Watch the signal in live data with the engine warm. It should stay steady between 0.6 and 0.8 volts.
3. Check the fuel trims. Very positive trims mean the mixture really is lean.
4. Inspect the manifold and the joints for black soot, which gives away the leak point.
5. Check the connector and wiring along their whole path under the vehicle.
6. Measure the sensor heater resistance.
7. Verify the circuit ground and its voltage drop.
8. Smoke test the intake if the trims point to a lean mixture.
9. Check the upstream sensor first. If it is controlling the mixture well and the downstream one is still low, the oxygen is getting in some other way.

How to fix the P0137 code

• Repair the exhaust leak you found by replacing the manifold gasket or welding the cracked joint.
• Replace the manifold studs if they are broken, a cause of blown gaskets that keep coming back.
• Repair an open signal circuit with solder and protection suited to the underside of the vehicle.
• Replace the sensor if its signal stays low with a sealed exhaust and a good circuit.
• Clean or replace the corroded connector and protect it with dielectric grease.
• Correct intake leaks or low fuel pressure if the mixture really is lean.
• Restore the circuit ground.
• Look for the air getting in before blaming a combustion problem when the upstream sensor is working correctly.

Technical reference data

• Expected signal with a healthy catalytic converter: steady between 0.6 and 0.8 volts.
• Threshold that sets the code: below 0.2 volts for a sustained period.
• An exhaust leak lets in air during the low-pressure pulses of the flow.
• The puffing is easiest to hear with the engine cold, before the metal expands.
• P0138 is the opposite code: voltage too high.
• Normal fuel trims: between minus 10 and plus 10 percent.
• A healthy converter leaves very little free oxygen in the gases, which is why the downstream sensor voltage is high and steady.

Brand-specific notes

Start with the exhaust, not the sensor. A leak ahead of it lets in air that the sensor reads as leftover oxygen, and replacing the sensor changes nothing because it is reporting correctly what reaches it.

Always listen with the engine cold. As it warms up, the metal expands and partly closes the leak, and the noise fades until it seems there was nothing there.

Frequently asked questions about P0137