P0155

What Does Code P0155 Mean?

DTC P0155 signifies an detected malfunction within the heater circuit of the Oxygen (O2) Sensor located in Bank 2, Sensor 1. Bank 2 refers to the engine bank that does not contain cylinder #1, and Sensor 1 denotes the upstream, pre-catalytic converter oxygen sensor. The Engine Control Module (ECM) or Powertrain Control Module (PCM) is responsible for monitoring the electrical integrity and operational status of the O2 sensor’s internal heating element. O2 sensors require a specific operating temperature, typically around 600-700 degrees Fahrenheit (315-370 degrees Celsius), to produce accurate voltage signals that reflect the oxygen content in the exhaust stream. To achieve this temperature rapidly during cold starts, and to maintain it under varying engine loads and exhaust gas temperatures, a dedicated heating element is integrated into the sensor. The PCM supplies voltage to this heater circuit and monitors its current draw, resistance, and voltage drop. When the PCM detects an electrical parameter (such as an open circuit, short circuit, or out-of-range resistance/current draw) that deviates from calibrated specifications for the Bank 2 Sensor 1 heater circuit, it registers P0155 and illuminates the Malfunction Indicator Lamp (MIL).

Common Symptoms

• Malfunction Indicator Lamp (MIL) Illumination: The “Check Engine” light will be illuminated on the dashboard.
• Reduced Fuel Economy: Without a properly functioning heater, the O2 sensor will take longer to reach operating temperature, leading to a prolonged open-loop fuel control phase during which the engine runs on a pre-programmed rich fuel mixture.
• Increased Exhaust Emissions: The inability of the O2 sensor to provide accurate data quickly results in sub-optimal air-fuel ratio control, leading to higher levels of pollutants.
• Poor Engine Performance (especially during cold starts): While less common than fuel economy issues, a malfunctioning O2 sensor can contribute to rough idle or hesitant acceleration until the engine and exhaust system naturally warm up.
• Potential for Other Codes: If the sensor never reaches proper operating temperature, it may lead to secondary catalytic converter efficiency codes (e.g., P0420/P0430) or general lean/rich condition codes (e.g., P0171/P0174 or P0172/P0175) over time.

What Causes the Code P0155?

• Faulty Oxygen Sensor (Bank 2 Sensor 1): The heating element within the sensor itself can fail due to age, thermal cycling, or internal electrical breakdown. This is the most common cause.
• Open or Short Circuit in the Heater Wiring: Damage to the wiring harness leading to the O2 sensor, such as abrasion, corrosion, rodent damage, or pinched wires, can create an open circuit (no continuity) or a short circuit to ground or power.
• Corroded or Loose Electrical Connector: Contamination (e.g., moisture, dirt, oil) or improper seating of the multi-pin connector at the O2 sensor can lead to high resistance or intermittent electrical contact, disrupting the heater circuit.
• Blown Fuse: While less common for an individual O2 sensor, a fuse protecting the heater circuit could blow, often due to an internal short within the sensor or wiring.
• Faulty Engine Control Module (ECM/PCM): Although rare, a defect in the PCM’s internal driver circuit responsible for supplying power or controlling the ground for the O2 sensor heater can cause this code.

How to Diagnose and Troubleshoot

Diagnosis of P0155 requires a systematic approach using an OBD-II scanner, digital multimeter (DMM), and visual inspection.

1. Visual Inspection:
   • Locate Bank 2 Sensor 1. This is typically on the exhaust manifold or close to it, on the side of the engine opposite cylinder #1.
   • Inspect the O2 sensor and its wiring harness for any obvious signs of damage, such as frayed wires, melted insulation, rodent chew marks, or severe corrosion.
   • Check the electrical connector for proper seating, bent pins, corrosion, or contamination. Disconnect and re-connect it to ensure good contact.
2. OBD-II Scanner Data Analysis:
   • Connect an OBD-II scanner and verify P0155 is present. Check for any related codes.
   • Access freeze frame data to note engine conditions (RPM, engine temperature, load) when the code was set.
   • If available, monitor live data parameters related to the O2 sensor heater circuit for Bank 2 Sensor 1, such as “O2S Heater Current,” “O2S Heater Voltage,” or “O2S Heater Duty Cycle.” A reading of 0V or 0A when the heater should be active indicates a problem.
   • Clear the code and perform a drive cycle, if possible, to see if the code immediately returns.
3. Digital Multimeter (DMM) Tests:
   • Heater Element Resistance Test:
     • Disconnect the O2 sensor electrical connector.
     • Identify the two wires responsible for the heater circuit (often two white or two black wires, consult a wiring diagram for specific vehicle).
     • Set your DMM to measure resistance (ohms).
     • Measure the resistance across the two heater pins on the sensor side of the connector.
     • Compare the reading to factory specifications (typically between 2 and 15 ohms at room temperature). An “OL” (open loop) or infinite resistance reading indicates an open circuit within the heater element, confirming a faulty sensor. Very low resistance (near 0 ohms) could indicate a shorted heater.
   • Power and Ground Supply Test (Harness Side):
     • With the O2 sensor disconnected and the ignition key in the ON position (engine OFF), identify the power supply wire and the ground wire for the heater circuit on the vehicle harness side of the connector (refer to wiring diagram).
     • Set the DMM to measure DC voltage.
     • Place the red lead on the power supply wire and the black lead on a known good chassis ground. You should read approximately 12 volts.
     • To test the ground circuit (if it’s a dedicated ground from the PCM), place the red lead on the 12V power supply wire (still on the harness side) and the black lead on the ground control wire from the PCM. You should read approximately 12V. If one side of the heater is switched via the PCM ground, you may need to crank the engine or activate the circuit via a bi-directional scanner to see the ground control.
   • Continuity Test (Harness Side to PCM):
     • If voltage/ground tests were inconclusive, or to rule out wiring issues, disconnect the vehicle battery and the PCM connector.
     • Use the DMM’s continuity function to check for continuity from the heater circuit wires at the O2 sensor harness connector back to their respective pins at the PCM connector. Also check for shorts to ground or power on these wires.
4. Fuse Inspection: Check the appropriate fuse in the vehicle’s fuse box that supplies power to the O2 sensor heaters. Replace if blown, but be aware that a blown fuse often indicates an underlying short circuit.

Recommended Repairs and Solutions

Once the diagnostic steps have pinpointed the root cause, the following repairs are typically recommended:

• Replace the Oxygen Sensor (Bank 2 Sensor 1): In the vast majority of P0155 cases, the O2 sensor itself, specifically its internal heating element, has failed. Ensure to replace it with a high-quality, OEM-grade or equivalent aftermarket sensor. Inferior sensors may not meet factory specifications, potentially leading to repeat failures or inaccurate readings.
• Repair or Replace Damaged Wiring/Connectors: If the diagnostic process identifies an open, short, or damaged connector in the wiring harness, the wiring should be meticulously repaired using proper soldering techniques, heat-shrink tubing, and appropriate gauge wire, or the entire sub-harness section should be replaced if damage is extensive. Replace corroded or broken connectors.
• Replace Blown Fuse: If a blown fuse was identified as the culprit, replace it with a fuse of the correct amperage. However, always investigate why the fuse blew, as it’s often a symptom of a short circuit elsewhere (e.g., in the O2 sensor itself or the wiring).
• PCM Replacement/Reprogramming: If all other components and wiring are confirmed to be in perfect working order, and power/ground signals are correctly supplied to the sensor (indicating an internal PCM driver fault), then PCM replacement or repair may be necessary. This is a very rare occurrence for O2 sensor heater circuit issues and should only be considered after exhaustive testing of all other possibilities.
• Post-Repair Verification: After performing any repair, clear the DTCs with an OBD-II scanner. Then, perform a comprehensive drive cycle under various conditions to allow the PCM to run all its diagnostic monitors and confirm that the P0155 code does not return and the O2 sensor heater monitor sets to “Complete.”