P0155: Definition and Causes of O2 Sensor Heater Circuit Malfunction (B2S1)
The diagnostic trouble code P0155 is a generic powertrain code (OBD-II) indicating a malfunction in the heater circuit of the pre-catalytic converter oxygen (O2) sensor on bank 2, sensor 1 (B2S1). While standardized, diagnosis and repair can vary significantly between different vehicle manufacturers.
Role of the Oxygen Sensor and its Heater Circuit
The oxygen sensor (lambda sensor) located upstream of the catalytic converter on bank 2 analyzes the oxygen content in the exhaust gases. This data is crucial; it allows the Engine Control Module (ECM) to adjust the air-fuel mixture in real-time for optimal combustion.
To function correctly, the sensor must reach a high temperature (approximately 600°C). The integrated heater circuit within the sensor allows it to reach this temperature quickly, especially when cold, thereby reducing the time to enter “closed loop” operation. This mode is essential for minimizing pollutant emissions at startup and optimizing fuel consumption.
The ECM continuously monitors the performance of the heater circuit. If, based on engine temperature, it detects that the sensor is taking too long to reach its operational temperature and begin transmitting a valid signal, it stores the P0155 trouble code and illuminates the Malfunction Indicator Lamp (MIL) on the dashboard.
Symptoms of Code P0155
Symptoms associated with this code can be subtle or more pronounced:
- Malfunction Indicator Lamp (MIL): The Check Engine light illuminates on the dashboard.
- Increased Fuel Consumption: A preheating fault keeps the engine in open loop longer, leading to a noticeable increase in gasoline or diesel consumption.
- Irregular Idle: The engine may have an unstable or rough idle.
- Reduced Performance: A slight loss of power or hesitation during acceleration may be noticeable.
- Increased Pollutant Emissions: The vehicle may fail emissions testing due to a non-optimal mixture.
It is important to note that sometimes the P0155 code may be stored without any obvious symptoms being noticeable to the driver, aside from the MIL illumination.
Potential Causes of the P0155 Fault
The P0155 code is specifically related to the sensor’s heater circuit. The causes are generally electrical:
- Internal Failure of the O2 Sensor: This is the most common cause. The heating element inside the lambda sensor is open or short-circuited. The sensor must then be replaced.
- Wiring Problem:
- Damaged, melted, cut, or corroded electrical wires connecting the sensor to the ECM.
- Oxidized, loose, or corroded connectors.
- Short circuit to ground or on the power supply circuit (+12V).
- Excessively high resistance in the circuit due to poor connections.
- Blown Fuse: The sensor heater circuit is often protected by a fuse. If this fuse has blown, the circuit is out of service.
- Engine Control Module (ECM) Failure: Although rare, a failure of the ECM itself can prevent control of the heater circuit.
Diagnosis and Solutions for Repairing Code P0155
Diagnosis requires a digital multimeter or a diagnostic scan tool to check vehicle parameters.
- Fuse Check: Always start by inspecting the fuse dedicated to the lambda sensor heater circuit in the fuse box.
- Visual Inspection: Examine the wiring and connectors between the B2S1 sensor and the ECM. Look for any signs of damage, corrosion, or burning.
- Electrical Test of the Sensor: Disconnect the O2 sensor connector. Measure the resistance of the heating element between the appropriate pins (usually the two wires of the same color, often white). Compare the measured value to the manufacturer’s specifications (often between 2 and 10 Ω). An infinite resistance (open circuit) or zero resistance (short circuit) confirms sensor failure.
- Circuit Test: Reconnect the harness side connector and check the supply voltage sent by the ECM to the heater circuit. A lack of voltage may indicate a wiring problem or an ECM issue.
Repair Solutions:
- Replace the blown fuse.
- Repair or replace damaged electrical wires and clean the connectors.
- Replace the faulty oxygen sensor: This is the most common solution. It is often recommended to use a quality OEM sensor.
Conclusion and Call to Action
The P0155 code, although precise in its indication (B2S1 sensor heater circuit), requires a methodical investigation to identify the root cause, which is electrical in the majority of cases. Ignoring this fault leads to increased fuel consumption and higher emissions.
⚠️ Warning: This information is provided for informational purposes only. Repairing a modern automotive system can be complex. If you are not comfortable with these diagnostic procedures, consult a professional mechanic equipped with advanced diagnostic tools. They can confirm the fault and perform the necessary repair to restore your vehicle to its optimal efficiency.