P0135
O2 Sensor Heater Circuit Bank 1 Sensor 1.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- 35
- FaultThe specific fault in that area
What it means
P0135 is the generic code for a fault in the heater circuit of the upstream oxygen sensor on bank 1, sensor 1, which sits before the catalytic converter on the side of the engine containing cylinder 1. The control unit monitors the heater's current draw or circuit state and sets the code when it sees a reading outside the expected range, such as an open circuit, a short or a heater resistance that is too high or too low. Without a working heater the sensor takes longer to reach operating temperature, so fuel control stays in open loop for longer.
Symptoms
- Engine management light on
- Poor fuel economy
- Slightly rough running or hesitation on a cold start
- Longer time before the engine settles into smooth closed-loop running
- Failed emissions test
- Possible additional oxygen sensor or fuel trim codes stored
Possible causes
- Failed heater element inside the upstream oxygen sensorlikely
- Damaged, chafed or corroded wiring to the sensor heater, including heater feed or groundlikely
- Poor connection at the sensor plug, with corrosion, bent pins or heat damagepossible
- Blown fuse or faulty relay supplying the oxygen sensor heater circuit (depends on model)possible
- Fault in the engine control unit heater driver circuitrare
- Exhaust leak near the sensor or contamination of the sensor causing sensor damagerare
How to diagnose
- Read the stored codes and freeze frame data, noting engine temperature, load and whether other oxygen sensor or fuel trim codes are present, then check for any manufacturer service bulletins for the model.
- Visually inspect the sensor wiring from the sensor to the plug for chafing, melting near the exhaust, or damage from previous repairs.
- Inspect the sensor connector for corrosion, moisture, loose or pushed-back pins and heat damage.
- Check the fuse and relay supplying the sensor heater, using the wiring diagram for the specific model and engine as the layout varies.
- With the ignition on and the connector unplugged, check the heater supply voltage and ground against the wiring diagram.
- Measure the heater element resistance across the correct sensor terminals and compare it with the manufacturer's specification for that sensor.
- Check the wiring for continuity and for shorts to ground or voltage between the sensor connector and the control unit.
- If the sensor, power supply and wiring all test good, check the control unit driver output and consider the control unit only after everything else is ruled out.
How to fix
- Replace the upstream oxygen sensor if the heater element resistance is out of specification or the circuit is open.
- Repair or replace damaged wiring, and properly repair any chafed or melted sections.
- Clean or replace the corroded or damaged connector terminals and plug.
- Replace the blown fuse after finding why it failed, or replace a faulty relay.
- Repair any exhaust leak near the sensor, then clear the code and confirm it does not return on a road test.
- Have the control unit repaired or replaced only if the driver circuit is confirmed faulty.
Can I keep driving?
The car can normally be driven for a short time, but book it in soon as fuel economy and emissions will suffer. Ignoring it may hide other faults and could lead to catalytic converter damage if the mixture runs rich for long periods.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.