Fault code P0135 – a step-by-step guide to diagnosing and fixing it

The P0135 code is triggered when there is a problem with the oxygen sensor heater circuit on cylinder bank 1. It appears after the engine control module has detected a short circuit or high resistance in the component's circuit. If the sensor doesn’t reach a certain temperature, it will not function properly. The longer it takes to warm up, the more difficult it will be for the engine to achieve the optimum operating conditions.

• Damaged cables or wires, obstructing the power supply network. The insulation sheath of the wires can fray or melt, resulting in a short circuit.
• A blown sensor fuse, caused by a power surge
• A faulty engine coolant temperature sensor
• A powertrain control module malfunction, although this is less common

• The engine runs rough, particularly while warming up
• The check engine comes on, indicating that there’s a problem
• The vehicle is less fuel efficient
• There’s change of smell and the exhaust system sounds strange

• To make sure that this isn’t a false error, you can reset the DTCs using an ODB2 scan tool. Take the car for a quick test drive to determine whether the fault is still there. If the check engine light comes on again, you will need to identify the underlying causes.
• Inspect the housing of the oxygen sensor and look for signs of contamination or damage. To do this, you will need to remove the device. It is possible to clean it using a specialised cleaning agent for air flow sensor. It may also be necessary to remove deposits and grime from the electrical connector pins around the device.
• You can visually inspect the wires and power supply cable for mechanical or thermal damage, however, it is advisable to check the voltage, even if the parts seem to be in a good condition. You can do this using a multimeter. If it doesn’t detect a voltage, you should check that this isn’t due to a blown fuse. The wires should always be examined first before the lambda device is replaced.
• You can also use the multimeter to test the resistance of the heater circuit. Compare the readings with the standard values specified by the manufacturer.
• Any defective components should be replaced. You may also want to check whether the engine coolant sensor is functioning properly.