P0137

What Does Code P0137 Mean?

Diagnostic Trouble Code P0137 indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a persistently low voltage signal from the Bank 1 Sensor 2 oxygen sensor circuit. Bank 1 refers to the engine bank that contains cylinder number one, and Sensor 2 designates the downstream or post-catalytic converter oxygen sensor. This sensor is primarily responsible for monitoring the efficiency of the catalytic converter by comparing the oxygen content in the exhaust gas before and after the catalyst. A normal, functioning downstream zirconia oxygen sensor should cycle its voltage output, typically between 0.1 to 0.9 volts, although it tends to show a relatively steady higher voltage (closer to 0.6-0.8V) if the catalytic converter is operating efficiently. When the ECM detects a voltage output from this sensor that remains consistently below a pre-calibrated threshold (often below 0.1-0.2 volts) for an extended period, it interprets this as an indication of either a severe lean condition in the exhaust gas, an open circuit, a short to ground within the sensor or its wiring, or a faulty sensor itself. This condition triggers the illumination of the Malfunction Indicator Lamp (MIL) and stores the P0137 code.

Common Symptoms

• Check Engine Light (MIL) Illumination: This is the most direct and common symptom.
• Failed Emissions Test: While the primary purpose of Sensor 2 is emissions monitoring, a consistently low voltage can lead to catalytic converter efficiency issues being masked or misrepresented, resulting in an inability to pass mandated emissions inspections.
• No Noticeable Drivability Symptoms: In many cases, P0137 will not cause any immediate or noticeable drivability problems as the downstream O2 sensor primarily monitors catalytic converter performance and does not directly influence fuel trim adjustments in the same way the upstream (Sensor 1) O2 sensor does.
• Slight Decrease in Fuel Economy: While uncommon, if the low voltage is due to an extreme exhaust leak, it could indirectly affect other sensor readings, leading to minor fuel economy impacts.

What Causes the Code P0137?

• Faulty Bank 1 Sensor 2 Oxygen Sensor: The most common cause, including internal sensor element degradation, heater circuit failure (though this often triggers a separate heater circuit code like P0141), or contamination leading to a compromised signal output.
• Wiring Harness Issues:
  • Open Circuit: A broken wire in the sensor signal or heater circuit.
  • Short to Ground: A signal wire inadvertently contacting chassis ground, causing a constant low voltage reading.
  • Corrosion: Poor electrical contact at the sensor connector or within the wiring harness due to moisture ingress or environmental exposure.
  • Chafed or Damaged Wiring: Physical damage to the wiring insulation leading to intermittent or constant shorts.
• Exhaust Leaks: A significant exhaust leak upstream of, or directly at, the Bank 1 Sensor 2 location can draw in ambient air, artificially increasing the oxygen content around the sensor. This can lead the sensor to report a false lean condition and a persistently low voltage.
• Contaminated Oxygen Sensor: Exposure to contaminants like silicone from RTV sealants, engine oil, or coolant can coat the sensor’s sensing element, reducing its ability to accurately read oxygen levels and causing it to report a low voltage.
• Faulty Engine Control Module (ECM/PCM): Although rare, an internal fault within the ECM/PCM that incorrectly interprets or processes the O2 sensor signal can trigger P0137. This should only be considered after all other possibilities have been thoroughly exhausted.

How to Diagnose and Troubleshoot

A systematic approach is crucial for accurately diagnosing P0137. Begin with simple checks and progress to more complex electrical diagnostics.

1. Visual Inspection:
   • Visually inspect the Bank 1 Sensor 2 oxygen sensor and its wiring harness from the sensor itself all the way back to the ECM/PCM connector. Look for signs of physical damage, chafing, melted insulation, corrosion at connectors, or loose connections.
   • Inspect the exhaust system around the sensor for any signs of leaks, such as black sooty areas. A smoke machine can be used to accurately pinpoint hard-to-find exhaust leaks.
   • Check for any indications of sensor contamination, such as oil, coolant, or excessive carbon deposits.
2. OBD-II Scanner Live Data Analysis:
   • Connect an OBD-II scan tool and monitor the live data stream for “O2S B1S2 Voltage.”
   • Observe the voltage reading. If the sensor is functioning correctly and the catalytic converter is efficient, the voltage should typically fluctuate slowly within a higher range (e.g., 0.6V to 0.8V). A consistently low reading, often below 0.2V, confirms the condition for P0137.
   • Note any other related pending or active codes, which might provide additional clues (e.g., O2 sensor heater circuit codes, misfire codes, or catalytic converter efficiency codes).
3. Digital Multimeter (DMM) Tests:
   • Sensor Heater Circuit Check: Disconnect the O2 sensor connector. Identify the heater wires (typically two white or similarly colored wires).
     • Check the resistance across the two heater wires. Refer to service manual specifications, but typically a few ohms (e.g., 2-10 ohms) is normal. An open circuit (infinite resistance) indicates a failed heater.
     • With the key ON, engine OFF, measure voltage at the harness side of the connector for the heater power supply wire (usually 12V relative to ground). Check for ground continuity on the heater ground wire.
   • Sensor Signal Circuit Integrity:
     • With the O2 sensor still disconnected, backprobe the signal wire (usually a black or purple wire) at the harness side of the connector while monitoring voltage. It should ideally be close to 0V or a small reference voltage (e.g., 0.45V) from the ECM.
     • Check for continuity of the signal wire from the sensor connector back to the ECM/PCM connector. Also, check the signal wire for shorts to ground or power.
     • Similarly, check the sensor ground wire for continuity to chassis ground and back to the ECM.
   • Simulated Sensor Output (Advanced): If accessible, and the sensor is heated, some technicians might briefly apply propane near the exhaust or create a rich/lean condition to see if the sensor responds by generating voltage. However, for a persistent low voltage, direct electrical testing of the circuit is usually more conclusive.
4. Exhaust Leak Confirmation: If a leak is suspected, use a smoke machine to inject smoke into the exhaust system while blocking the tailpipe. Smoke escaping near the B1S2 sensor confirms an exhaust leak that could cause the false lean condition.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace the Bank 1 Sensor 2 Oxygen Sensor: If the sensor itself tests faulty (e.g., open heater, no voltage response during testing, or confirmed contamination), replacement with a high-quality OEM or equivalent aftermarket sensor is the most common solution. Ensure the correct type of sensor (e.g., zirconia, titania, wideband) is used for the specific application.
• Repair or Replace Wiring Harness: If damaged wiring, corroded connectors, or an open/short circuit is identified, perform meticulous repairs. Use appropriate connectors, soldering techniques, and heat-shrink tubing to ensure a durable, weather-tight repair. In cases of extensive damage, replacing a section of the harness or the entire pigtail may be necessary.
• Repair Exhaust Leaks: Weld, patch, or replace the affected section of the exhaust system to eliminate any leaks near the Bank 1 Sensor 2. Ensure proper gasket seating and torquing of exhaust components.
• Address Contamination Sources: If the sensor is contaminated, identify and rectify the source. For example, if silicone is present, ensure RTV sealants used in engine repairs are oxygen sensor safe. If oil or coolant is fouling the sensor, diagnose and repair the engine issue causing the leak (e.g., valve cover gasket, head gasket).
• ECM/PCM Replacement (Rare): Only after all other diagnostic steps have definitively ruled out the sensor, wiring, and exhaust issues, and with conclusive evidence of an internal ECM/PCM fault, should module replacement be considered. This often requires professional programming.

After any repair, clear the diagnostic trouble codes using a scan tool. Perform a comprehensive drive cycle to allow the ECM/PCM to complete its internal diagnostic monitors and confirm that the P0137 code does not reappear and the MIL remains off.