P0135
O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
The heating element inside the upstream oxygen sensor draws no current, draws too much current, or is open-circuit. The heater brings the sensor to operating temperature so closed-loop control can start within seconds rather than minutes.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
Symptoms
- Check engine light on after several drive cycles
- Slightly increased fuel consumption during cold start
- Delayed entry into closed-loop fuel control
- Failed emissions inspection (lambda readiness)
Common causes
Ordered by how often each turns out to be the cause.
- Heater element open-circuitCommon
- Heater fuse blownPossible
- Wiring fault to heaterLess likely
Parts involved
- Oxygen Sensor Upstream
- Oxygen Sensor Heater Relay
- Oxygen Sensor Heater Fuse
P0135 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.