The onboard diagnostic (OBD) code P0135 is an oxygen sensor heater circuit malfunction in bank 1, sensor 1. This code indicates that the Engine Control Module (ECM) has tested the oxygen sensor heater circuit and has found a problem with sensor 1 in bank 1. Bank 1 refers to the side of the engine that has the #1 cylinder. Bank 2 would be the opposite side of this. This is important to understand in order to find the location of the malfunctioning oxygen sensor heater circuit.

The ECM tests the oxygen sensor heater circuit at startup to check for one of three things – excessive current draw, opens, or shorts. If the ECM detects an open circuit, a short-circuit, or excessive current draw in the heater circuit, it will set the P0135 code.

The ECM also monitors how long it takes the sensor to warm up so it can send an adequate signal. If the sensor takes too long to warm up, the code will be triggered.

Common Symptoms

  • Check engine light will be lit
  • Poor fuel economy is possible

Possible Causes

  • Faulty heated oxygen sensor in bank 1, sensor 1
  • Heated oxygen sensor blown circuit fuse
  • Heated oxygen sensor circuit open, shorted to ground
  • Heated oxygen sensor circuit poor electrical connection
  • Faulty ECM

What to Check

  • First, visually check the electrical connections and wiring harness to the bank 1, sensor 1 oxygen sensor.
  • Check for water getting into the heated oxygen sensor connector, which can cause a blown fuse.
  • Test the oxygen sensor connector for the correct input voltage from the ECM for the heater circuit.
  • Check the oxygen sensor heater circuit for the right resistance within vehicle specifications.
  • Also, check for the condition of the heated oxygen sensor fuse and connectors. If these are all fine, replacing the oxygen sensor will typically solve the problem.

How to Fix It

First, test the heater circuit wiring using a multimeter. Disconnect the oxygen sensor connector and test the heater circuit for power and ground. With the black lead connected to ground and the other to the connector, you should see a reading of about 12 volts. If not, it means there is a problem with the sensor’s power supply. Use a wiring diagram for your specific vehicle to find where the circuit fault is.

To test ground, connect the red lead to the positive battery terminal and the black lead to ground. Again, you should see a reading of around 12 volts. If not, check your vehicle’s wiring diagram to find out where the ground circuit fault is.

If power and ground are established, the second step is to test the sensor heating element for either an open circuit or high resistance. With your multimeter on the ohms setting, connect both leads to the heater circuit pins on the sensor side of the connector and see if the resistance value is within specifications for your vehicle. If it’s not, it means that the heating element inside the sensor has too high of a resistance and the oxygen sensor should be replaced.

If while performing this test, your multimeter shows a reading of OL, it means there is an open circuit and the solution is the same as above – the sensor should be replaced.

Additional Tips

Remember that bank 1 refers to the side of the engine with the #1 cylinder, no matter what the vehicle or engine type.

If the electrical connections are all sound, then replacing the oxygen sensor will typically solve the problem.

If water is discovered in the connector, then replace the fuse to the heater circuit.