Code P0135 - Heated Oxygen Sensor Heater Circuit Malfunction (Bank1 Sensor1)

The air/fuel ratio (A/F) sensor also known as a front oxygen sensor (Bank 1 Sensor1) is installed in the exhaust before the catalytic converter. The sensor is heated by an electric heating element built inside the sensor. This is needed to quickly warm up the sensor to normal operating temperature when the car is started. The code P0135 means there is a malfunction in the A/F sensor heater circuit. Read more about the A/F sensor.

• Symptoms
• Causes
• Diagnostic
• Common problems causing the code P0135
• Sample step-by-step code P0135 diagnostics


In most cases, there is no symptoms other than the Check Engine Light. Often the Check engine light may come on after the vehicle has been started cold.


Common causes include:
- Faulty air fuel ratio (A/F) sensor also known as front oxygen sensor or Bank 1 Sensor 1
- Corrosion or damaged terminal at the A/F sensor connector
- Damaged or shorted wiring between the sensor and the PCM or between the sensor and the fuse box.
- Blown or missing fuse for the sensor heater circuit. See: How to check a fuse.
- Aftermarket or incorrect air fuel ratio sensor is installed
- Low battery charge.

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How the code P0135 is diagnosed:

The code P0135 is a pure electric code and is easy to diagnose using a multimeter. In many cars, the battery voltage is supplied through a fuse and relay to the sensor heating element. The ground for the heating element comes from the PCM. A typical diagnostic procedure involves checking the sensor heater fuse, then the relay, then the resistance of the A/F sensor heating element. See the example of step-by-step diagnostic with photos for the code P0135 on the next page. Correct diagnostic procedure can be found in the service manual for your vehicle. We posted several websites that provide access to a service manual for a subscription fee at the bottom of this page.

Here are a few common problems known to cause the code P0135 in different cars:

In some Acura, Honda, Lexus and Toyota vehicles, the failed sensor heating element is known to cause this code. A failed A/F sensor (Bank 1 Sensor 1) can be confirmed by measuring the resistance of the heating element of the sensor.
The resistance should be low, typically between 0.9 and 10 ohms, depending on the vehicle. Specifications for different cars can be found in the service manual. Replacing the sensor often solves the problem.

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