P0135 — O2 Sensor Heater Circuit (Bank 1, Sensor 1)
Low severityWhat it means
The heater inside the upstream oxygen sensor on bank 1 is not working. The sensor must be hot to produce a usable signal; without its heater, it warms up late and the engine runs longer on a fixed fuel map after every start, wasting fuel.
Common symptoms
- Check engine light
- Slightly higher fuel consumption
- Rough running for a while after cold starts
- Failed emissions inspection
Likely causes
- Failed heater element inside the aged sensor
- Blown heater circuit fuse
- Damaged wiring or connector
- Rare: driver circuit fault in the engine control unit
What to check first
- Check the heater circuit fuse first
- Inspect the sensor connector and wiring for damage or chafing
- Measure the heater resistance across the heater pins
- Replace the sensor if the element is open-circuit
Can you keep driving?
Safe to drive; fuel economy and emissions suffer slightly until fixed.
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