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

Symptoms

• Check engine light
• Increased fuel consumption, especially on cold starts
• Slight rough running until the engine warms up
• Slow to enter closed-loop operation

Common causes

• Failed heater element inside the upstream O2 sensor
• Blown O2 heater fuse
• Damaged wiring or melted connector
• Open circuit in the heater control line

What to do?

Check the heater fuse and the connector at the sensor first. Measure the heater resistance with a multimeter (compare to spec - usually a few ohms). If wiring and fuse are good but resistance is out of range, replace the upstream O2 sensor on bank 1.