What it means
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.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 sensor 1 is the upstream oxygen sensor, and its heater warms the sensing element so the ECU can use it soon after a cold start. P0135 sets when the ECU detects an electrical fault in that heater circuit, such as an open element or abnormal current.
Common causes
Most likely first.
- Heater element open-circuit
- Heater fuse blown
- Wiring fault to heater
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)
How to diagnose it
- Record freeze-frame data and check whether P0135 returns after a cold start, when the upstream sensor heater is needed most.
- Inspect bank 1 sensor 1's heater wiring and connector near the exhaust for heat damage, chafing, corrosion, or loose terminals.
- Check the relevant heater fuse and power supply using the vehicle's wiring information.
- Measure heater-element continuity and test the control and supply circuits against the manufacturer's specification.
- If power, wiring, and control are correct, replace the upstream sensor only if its heater fails the specified test.
Common fixes
- Repair heat-damaged or chafed heater wiring.
- Clean and secure the bank 1 sensor 1 connector.
- Replace a blown heater fuse after checking for a short that caused it to fail.
- Replace bank 1 sensor 1 if its heater element is open or otherwise outside specification.
Can I keep driving?
The vehicle is usually drivable if it runs normally, but cold-start fuel control may be less efficient and emissions readiness may remain incomplete. Arrange repair soon, and avoid extended use if the engine runs poorly or the warning is accompanied by other faults.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.