P0137 fault code symptoms, causes, diagnosis, repair

What does the P0137 code mean?

The P0137 is a generic OBD-II powertrain trouble code. Its official definition is “O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)“.

This diagnostic trouble code is stored by the vehicle’s Engine Control Module (ECM) when it detects that the signal voltage from the downstream oxygen sensor (Bank 1, Sensor 2) is persistently and abnormally low. “Bank 1” refers to the side of the engine containing cylinder number 1, and “Sensor 2” is the post-catalytic converter oxygen sensor. This sensor monitors the oxygen content in the exhaust gases *after* they have passed through the catalytic converter. A properly functioning downstream sensor typically produces a relatively stable, high voltage signal (often between 0.6V and 0.8V) when the catalytic converter is working efficiently. Code P0137 triggers when the ECM sees this signal stuck at or repeatedly dropping to a very low voltage threshold (typically around 0.1V-0.2V), which it interprets as a fault in the sensor circuit. This fault prevents the ECM from accurately monitoring catalytic converter efficiency. The first diagnostic step is to connect a professional obd2 scanner to confirm the code and examine the sensor’s live data.

Symptoms of Error Code P0137

• Illuminated Check Engine Light (CEL).
• You will very commonly also see a catalytic converter efficiency code, such as P0420 for Bank 1, because the low voltage signal falsely indicates high oxygen content after the catalyst.
• The vehicle will fail an emissions test.
• Emissions readiness monitors will be incomplete.
• Typically, there are no drivability symptoms like rough idle or hesitation. The downstream sensor does not control fuel mixture.

Main Causes of Error Code P0137

• Faulty Downstream Oxygen Sensor (Bank 1, Sensor 2): The most common cause. The sensor’s sensing element has failed, causing it to output a continuously low signal voltage.
• Short to Ground in the O2 Sensor Signal Wire: The wire carrying the sensor signal back to the ECM is frayed and touching the chassis or exhaust, pulling the voltage down to near 0V.
• Exhaust Leak After the Catalytic Converter: A leak in the exhaust pipe near or after the downstream sensor allows unmetered air (oxygen) to enter. This causes the sensor to correctly read a lean condition, sending a low voltage signal. The ECM may interpret this persistent lean signal as a circuit fault.
• Contaminated Oxygen Sensor: The sensor tip may be contaminated by coolant, oil, or silicone, skewing its reading low.
• Wiring/Connector Issue (High Resistance or Open in Reference Circuit): A problem with the sensor’s 5V reference circuit or a poor ground can result in a low signal reading.
• Failing Catalytic Converter (Rare as a *direct* cause of P0137): A severely degraded catalytic converter that is no longer storing oxygen could theoretically lead to a higher oxygen content downstream, resulting in a lower sensor voltage. However, this would typically set a P0420 code first, and the voltage might not be low enough to trigger a P0137 circuit fault.

How to Diagnose Error Code P0137?

1. Initial Scan and Live Data Review: Use a professional obd2 diagnostic scanner to read code P0137 and any related codes (especially P0420). Access live data and monitor the voltage for “Bank 1, Sensor 2.” A key observation: If the voltage is stuck at or very near 0.1V and does not change, it suggests a hard circuit fault (short to ground) or dead sensor. If it fluctuates but averages very low, it may indicate a real condition like an exhaust leak.
2. Check for Exhaust Leaks: This is a critical step. With the vehicle safely raised, start the engine and carefully listen for a hissing or ticking sound from the exhaust pipe behind the catalytic converter. Inspect for soot trails, rust holes, or loose connections at the flanges.
3. Visual Inspection of Sensor and Wiring: Locate the downstream O2 sensor. Inspect its wiring pigtail for damage where it may have contacted the hot exhaust or road debris. Check the connector for corrosion.
4. Test the O2 Sensor Circuit for Short to Ground: With the sensor connector disconnected and the ignition ON, use a digital multimeter to check the voltage on the signal wire (at the vehicle harness side). It should be very low (less than 0.1V). If it reads 0.5V or shows reference voltage, the circuit from the ECM is likely okay. Next, with the ignition OFF, check for continuity between the signal wire pin (harness side) and a clean chassis ground. Continuity indicates a short to ground in the wire.
5. Simulate a Rich Condition: To test the sensor’s responsiveness, create a temporary rich condition. With the engine at operating temperature and your vehicle diagnostics tool displaying the downstream O2 voltage, have an assistant briefly press and hold the accelerator to about 2500 RPM. A functioning downstream sensor should eventually show a slow increase in voltage. No response indicates a faulty sensor.
6. Check the Sensor’s Heater (if in doubt): A failed heater (code P0141) can cause a sensor to not reach temperature, potentially affecting the signal. Test heater resistance as a secondary check.

Possible Causes and Diagnostic Methods

Tools needed: A professional obd2 diagnostic tool with live data, a digital multimeter, jack and jack stands, basic hand tools, and possibly a smoke machine for leak testing.

How do I fix error code P0137? (Solutions to the Problem)

Simple Fixes

• Repair Exhaust Leaks: Weld a small hole, replace a gasket, or tighten loose clamps/bolts on the exhaust pipe after the catalytic converter.
• Clean and Re-seat Electrical Connectors: Disconnect the battery. Clean the downstream O2 sensor connector with electrical contact cleaner. Ensure a tight, corrosion-free connection.
• Repair a Short to Ground in Wiring: If a short is found in an accessible section of the signal wire, repair the insulation with solder and heat-shrink tubing.

In-depth Diagnosis and Repair Solutions

• Replace the Downstream Oxygen Sensor (Bank 1, Sensor 2): This is the solution if the sensor is faulty and a short circuit is ruled out. After installation, clear all codes with your car code reader. The P0420 code, if present, may clear after several drive cycles with the new, functioning sensor.
• Repair Damaged Vehicle Wiring Harness: For extensive damage to the main harness under the vehicle, a professional repair may be needed.

Fix faults based on symptoms

Common Error Code P0137 in Vehicles

P0137 is common, often appearing alongside P0420. It is frequently seen in:

• Older Toyota/Honda/Nissan vehicles: Due to aging exhaust systems (leaks) and eventual sensor failure.
• Trucks and SUVs (Ford, GM, Dodge): The longer exhaust systems and exposure to road elements make exhaust leaks and wiring damage common.
• European vehicles (VW, Audi, BMW): The emissions systems are sensitive, and any exhaust leak or sensor degradation will trigger this code. A capable vehicle diagnostics tool is very helpful for diagnosis.

P0137 Frequently Asked Questions

Is it safe to drive with a P0137 code?

Yes, from a drivability and safety perspective, it is generally safe. The downstream sensor does not affect engine performance. However, you are driving with an unresolved emissions fault and a non-monitored catalytic converter. It should be repaired to pass inspection and prevent potential catalyst damage if an underlying rich condition exists.

What is the difference between P0136 and P0137?

P0136 is a general “O2 Sensor Circuit” malfunction for the downstream sensor. P0137 is more specific: “O2 Sensor Circuit Low Voltage.” P0137 indicates the ECM has determined the signal is consistently too low, pointing more directly to a short to ground, a dead sensor, or an exhaust leak.

Can a P0137 code cause a P0420 code?

Yes, absolutely. In fact, it’s the most common scenario. A faulty downstream sensor providing a falsely low voltage (P0137) makes the ECM think the catalytic converter is not storing any oxygen (i.e., inefficient), so it also sets a P0420 code. Fixing the sensor or the exhaust leak causing P0137 will often clear the P0420 code.

How much does it cost to fix P0137?

Costs vary: Diagnosis: $80-$150. A new downstream O2 sensor: $80-$250. Labor: 0.5-1 hour ($50-$100). Repairing an exhaust leak may cost $100-$400. Total can range from $150 for a simple sensor swap to $500+ for exhaust repair.

Do I need a special scan tool to diagnose P0137?

A basic obd reader can confirm the code. However, a professional obd2 diagnostic scanner that displays live O2 sensor voltage data is essential for differentiating between a faulty sensor (stuck voltage) and an exhaust leak (low but possibly varying voltage). A multimeter is needed for circuit testing.

Will disconnecting the battery clear P0137?

It may temporarily clear the code and light. However, if the faulty sensor, wiring short, or exhaust leak is still present, the code will return within one or two drive cycles as the ECM retests the system. The root cause must be fixed. Use an obd tool scanner to clear codes after repair.

P0137 Related OBD2 Errors

• P0136: O2 Sensor Circuit (Bank 1, Sensor 2)
• P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
• P0140: O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)
• P0141: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
• P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Important: Code P0137 is a circuit low voltage fault for the downstream oxygen sensor. Diagnosis should prioritize checking for exhaust leaks after the catalytic converter, as this is a very common and fixable cause. Do not automatically replace the sensor or catalytic converter. Use a professional vehicle diagnostics tool to analyze the live sensor data and a multimeter to check for electrical shorts. Correctly diagnosing and repairing P0137 can often resolve a companion P0420 code, preventing an unnecessary and costly catalytic converter replacement.