P0135 – O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

Indicates a problem with the heater circuit for the upstream oxygen sensor on Bank 1.

Plain‑English Explanation

Modern oxygen sensors use an internal heater to bring the sensor up to operating temperature quickly. The ECU monitors current draw and response time from the heater circuit. If it detects an open circuit, short or poor performance on Bank 1 Sensor 1, it sets P0135.

Common Symptoms

  • Check engine light on.
  • Sometimes slightly increased fuel consumption.
  • Possible delay in closed-loop operation after cold start.

Typical Causes

  • Failed O2 sensor heater element.
  • Broken wiring or corroded connector at the sensor.
  • Blown fuse or relay feeding the heater circuit.
  • In rare cases, ECU driver fault.

What To Check First

  2. Visually inspect the sensor wiring and connector for damage or corrosion.
  4. Check relevant fuses and relays for the O2 heater circuit.
  6. Measure resistance of the heater circuit according to service data.
  8. If wiring and power/ground are good, replace the sensor.