Code P0135: Malfunction in O2 Sensor Heater Circuit (Bank 1, Sensor 1)
Fault code P0135 is a generic OBD-II powertrain code that indicates a problem with the upstream oxygen sensor heater circuit (bank 1, sensor 1). This circuit is crucial for reducing the sensor’s warm-up time, allowing it to reach its optimal operating mode (“closed loop”) more quickly and provide accurate readings to the vehicle’s ECU/ECM.
Function of the O2 Sensor Heater Circuit
The oxygen sensor (lambda probe) measures the oxygen content in the exhaust gases. Its integrated heating element allows it to reach its operating temperature (approximately 600°C) quickly after a cold start. If the ECU detects, based on the coolant temperature, that the sensor is taking too long to start switching (sending a signal), it records the fault code P0135.
Symptoms of Code P0135
- The Malfunction Indicator Lamp (MIL) illuminates on the dashboard.
- Increased fuel consumption (decreased efficiency).
- Unstable idle or poor engine performance (especially during cold starts).
- Potentially higher pollutant emissions.
- In some cases, no noticeable symptoms other than the MIL light.
Possible Causes of Fault P0135
Several components can be the source of this code:
- Faulty oxygen sensor: The internal heating element is burnt out (open circuit or short circuit).
- Wiring problem: Damaged, corroded, loose, or short-circuited wires between the sensor and the ECU.
- Faulty connector: Poor contact, oxidation, or damage on the O2 sensor connector.
- Blown fuse: The sensor heater circuit is often protected by a fuse.
- Problem with the ECU (less common): Failure of the engine control module.
Diagnosis and Solutions for Repairing Code P0135
Before replacing parts, a methodical check is essential.
- Check the fuse: Consult the owner’s manual to locate and inspect the fuse dedicated to the O2 sensor heater circuit.
- Visually inspect the wiring and connectors: Look for any signs of damage, corrosion, or burning.
- Test the resistance of the heating element: Using a multimeter, measure the resistance between the heater pins of the O2 sensor (usually the two pins of the same color). Compare the measured value (generally between 2 and 20 Ω) to the manufacturer’s specification. An infinite resistance (open circuit) or zero resistance (short circuit) confirms a faulty sensor.
- Check the circuit voltage and ground: With the ignition on, check that the supply voltage (12V) reaches the connector and that the ground is good.
Repair Solutions:
- Repair the wiring: If a wire is cut or short-circuited, repair it or replace the damaged section.
- Clean or replace an oxidized or damaged connector.
- Replace the oxygen sensor: If the internal heating element is faulty, replacing the sensor is the only solution. Use a quality part.
- Replace a blown fuse (after identifying and correcting the cause of the short circuit that blew it).
Important Tip: After any repair, clear the fault codes with an OBD2 scanner and perform a road test to verify that code P0135 does not reappear.
Need Help Diagnosing Your Vehicle?
Code P0135 is generally simple to diagnose with the right tools. If you are not comfortable with these steps, consult a professional mechanic for an accurate diagnosis and reliable repair. Avoid driving for a long time with this fault to preserve your fuel consumption and the proper functioning of the engine.