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

What is P0137?

Diagnostic trouble code P0137 is defined as “O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2).”

It indicates a low voltage in the downstream oxygen sensor circuit (rear O2 sensor on Bank 1) — which measures the air fuel ratio from the catalytic converter. The trouble code is triggered when the Engine Control Module detects a voltage output of less than 210 mV for more than 20 seconds.

Typically, the downstream oxygen sensor should produce a steady voltage of about 450 mV. And when it’s lower, it means there’s excessive oxygen in the exhaust stream or a problem with the sensor circuit.

The problem could be due to several reasons, including wiring damage, high resistance, or even a faulty sensor. And its impact can mean severe performance issues.

Common symptoms

Here are the signs that may accompany a low voltage signal in the downstream O2 sensor circuit (DTC P0137):

• Engine running rich: A low voltage will be followed up by the Engine Control Module adjusting the air fuel ratio, leading to your engine running rich. This usually happens during the testing of the sensor. 
• Loss of engine power: The improper air fuel mixture or high oxygen content can lead to the engine losing power while running. It can result in engine overheating, slow acceleration, a rough idle, or even engine failure. 
• Exhaust leak and odor: A possible symptom of a P0137 code is an exhaust leak before the O2 sensor or after the catalyst. Moreover, you may notice a strong odor from the exhaust stream. 
• Flashing Check Engine Light: When the Engine Control Module detects low voltage in the downstream O2 sensor, it automatically triggers the Check Engine Light.

Can I still drive?

Yes, you can drive with a trouble code P0137, though it isn’t recommended or good for your vehicle.

The issue is moderately serious, and extended driving can cause:

• Engine failure and damage
• Catalytic converter damage
• Poor fuel mileage
• Failed emission tests
  

The damage to your engine bay components will keep increasing as you use the car, eventually leading to permanent engine damage and high repair costs. So, reach out to a mechanic ASAP.

P0137 causes

Here are potential causes of a code P0137: 

• Bad oxygen sensor (including a faulty heated oxygen sensor circuit)
• Short circuit in the oxygen sensor circuit due to high voltage
• High resistance or open circuit in the O2 signal circuit
• Exhaust leak before the sensor
• Plugged catalyst
• Low fuel pressure or fuel pump regulator issues
• Engine running lean
• Engine misfires 
• Outdated ECM software (rare)

Diagnosis

Here are the steps a mechanic would take to diagnose a code P0137:

1. Scan the code using an OBD-II scan tool, document the freeze frame data (failure records), and clear the codes to verify the low voltage condition.
   
2. Monitor the downstream sensor data and compare it with the upstream sensor voltage data.
   
3. Check the wiring, harness, and harness connector of the second oxygen sensor circuit for damage or corrosion.
   
4. Look for an exhaust leak before the rear sensor. They may also examine the vacuum line for a vacuum leak.
   
5. Check the fuel pressure to see if it’s low using a fuel pressure gauge.
   
6. Follow other pinpoint tests mentioned in the manufacturer’s manual.
7. An additional step would be checking the O2 sensor for fluid contamination or physical damage, which could help identify the reason behind the faulty sensor.

Note: If a mechanic detects other pending codes besides a P0137, they’ll resolve those codes before installing a new sensor.

Possible repairs for P0137 & Costs

Here’s how a mechanic may resolve a DTC P0137:

1. They’ll repair the O2 sensor wiring, including the harness and harness connector if there’s damage or corrosion. This includes repairing issues due to a short circuit or high resistance in the O2 sensor circuit. 
2. The mechanic would repair any leak in the exhaust manifold. 
3. If the fuel pressure regulator is faulty and causing the low voltage readings, a mechanic would replace it since it would mean the O2 sensor is functioning fine.
4. The mechanic will repair the vacuum line if there’s a leak. 
5. They’ll replace the Bank 1 Sensor 2 O2 sensor if it’s faulty. But if the cause is a restricted or damaged catalytic converter, they’d replace that instead. Sometimes, the rear O2 sensor could be damaged due to a broken catalyst, and both would need to be replaced.

After doing any of these repairs, your mechanic will re-scan the codes using the scan tool or check for flashing Check Engine Light to see if the issue has been resolved.

Note: A mechanic would probably prioritize addressing any sensor heater issues or any pending codes related to it since it could also trigger the low voltage code in the O2 sensor.

Repair Costs:

Since the best fix for a P0137 code isn’t always a new sensor, it’s improbable that your mechanic would give a repair estimate without conducting a diagnosis.

Typically, a diagnosis would cost between $75-$150, depending on the mechanic’s labor rate. Once you know the potential cause, the cost of repairs can depend on the replacements or repairs needed. 

Here are the estimated costs of various repairs and replacements (including the cost of the part and labor):

• Oxygen Sensor (Rear Sensor): $200-$300
• Exhaust Repair (Welding): $100-$200
• Fuel Pump: $1300-$1700
• Fuel Pressure Regulator: $200-$400
• Vacuum Leak: $100-$200
• Catalytic Converter: $800-$2500