What does the P0135 OBD-II code mean?

O2 sensors are heated and contain heating elements to help them achieve operating temperature quickly. When the OBD-II trouble code P0135 is activated, it is indicating that the engine control module (ECM) has detected a problem with the O2 sensor heater circuit bank 1 sensor 1. O2 sensors are typically mounted directly onto the exhaust pipe near the catalytic converter, giving feedback to the ECM to control the injectors correctly. When a P0135 code is activated, the ECM remains in an open loop, meaning reduced fuel economy until the exhaust gets hot enough to activate the O2 sensor into giving voltage output.

How serious is the P0135 code?

The P0135 diagnostic code is deemed moderately serious and is not an issue that requires immediate attention. However, whilst it is acceptable to keep driving a vehicle with this particular OBD-II code activated, the problem should be resolved as expeditiously as possible. By continuing to drive, it could result in more costly repairs in the future. Similarly, not resolving the issue means the ECM stays in an open loop until the problem is repaired or the O2 sensor starts working again, meaning the engine will run extra rich, burn more fuel and will begin to build carbon.

What are the common symptoms of a P0135 trouble code?

  • The Check Engine light illuminates
  • The ECM will not receive any O2 sensor feedback until the O2 sensor starts to work again or is repaired
  • Decreased fuel economy whilst the ECM remains in an open loop
  • The engine may generally run rough

What are the causes of a P0135 OBD-II code?

  • Faulty wiring/connections
  • Short or open ground in the wiring
  • Blown Fuse
  • Defective pre-catalyst oxygen sensor
  • Engine coolant temperature sensor is faulty
  • Engine control module (ECM) is not operating correctly

How to diagnose a P0135 OBD-II code?

The easiest way to diagnose an OBD-II fault code is to use an OBD-II scanner or book a diagnostic check with a trusted mechanic or garage. The main point to remember is to ensure the problem doesn’t lie with defective wiring and connections before replacing the oxygen sensor:

  • Check to see if there are any technical service bulletins available for your vehicle’s make and model to see if there is a known problem for your car that can help resolve the issue.
  • Scan your system for other OBD-II codes.
  • Clear the fault codes and take the car for a test drive to verify failure. If the code persists: 
  • Make a visual inspection of the electrical connections and wire harness to the bank 1 sensor 1 O2 sensor, including checking the sensor for contaminants and water ingress into the harness cover.
  • Check the engine ground for corrosion or loose connections.
  • Check and monitor O2 sensor data to see if the heater circuit is working, including testing the O2 sensor connector for the heater circuit’s proper input voltage from the ECM.
  • If the O2 sensor has no power, check the fuse.
  • Finally, follow the manufacturer’s specific pinpoint tests for further diagnosis and possible ECM failure.

How to fix a P0135 OBD-II code?

It’s advisable to test drive the vehicle after each check or repair is performed to see if the fault code clears. If the code returns, move on to the next repair. The most common repairs to resolve a P0135 trouble code are:

  • Verify the code with an OBD-II scanner, and reset the fault code. Follow this with a test drive of the car to see if the trouble code clears. If it returns:
  • Make a visual inspection of the electrical connections and wire harness to the bank 1 sensor 1 O2 sensor and repair or replace the connectors or wiring as necessary.
  • If the O2 sensor heater circuit has no power, replace the fuse (after any short is corrected).
  • Replace the O2 sensor.
  • Replace the ECM.

How to avoid a P0135 Code?

This particular diagnostic code is typically triggered by wiring, connections or parts failing, so it’s not always possible to avoid the problem from happening. However, it always makes sense to maintain and service your car regularly, including changing oil and filters and visually checking for problems with wiring and connectors. By undertaking these basic but necessary steps, you will ensure your vehicle and engine remains in good running order.

About Fixter

Fixter is revolutionising the car maintenance industry, one repair at a time. Fixter was founded to make car maintenance as easy as booking a taxi. Digital, transparent and stress-free, with world-class customer service. Since launching in Manchester in 2017, Fixter has expanded to more than 100 towns and cities across the UK and provided thousands of car owners with honest, convenient and affordable car repair services.