P0135 Code: O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
The heater inside the upstream oxygen sensor on bank 1 isn't working or its circuit is faulty.
At a glance
Severity: Low
- Meaning
- O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $1–$400
- DIY difficulty
- Easy
An oxygen sensor only produces a usable signal once its tip is several hundred degrees hot, so it has a built-in electric heater that gets it there within a minute or so of a cold start. The PCM powers that heater and monitors its current or resistance; if the values are out of range, or the sensor takes far too long to become active, it sets P0135. Bank 1 Sensor 1 is the upstream sensor ahead of the catalytic converter on the side of the engine with cylinder 1; four-cylinder engines have only one. In most cases the heater element inside the sensor has simply burned out, but a blown fuse or a damaged wire near the hot exhaust gives the same code. Many newer vehicles report the same fault with P0030 to P0032 instead.
Can I keep driving? Safe to drive. Until the heater works, the engine stays in open-loop fueling longer after each cold start, so mileage suffers on short trips and the car won't pass an emissions test.
How to read P0135
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 35 Specific fault
Symptoms
- Check engine light on
- Slightly worse gas mileage, mostly on short trips
- Engine takes longer to enter closed-loop after a cold start
- Oxygen sensor heater monitor shows incomplete or failed
- Usually no change in how the car drives
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Heater element burned out inside the upstream O2 sensor
- Melted, chafed or broken wiring where the harness runs near the exhaust
- Blown O2 heater or engine control fuse
- Corroded or loose connector at the sensor
- Universal replacement sensor spliced in incorrectly
- PCM heater driver or heater relay fault
Diagnosis step by step
Identify the correct sensor
Bank 1 Sensor 1 is the oxygen sensor before the catalytic converter on the cylinder 1 side of the engine. On a V6 or V8, check a service diagram, because replacing the wrong upstream sensor is a common and costly mistake.
Tools: Flashlight
Check the fuses
Look up the fuse that feeds the O2 sensor heaters (often labeled O2, HTR or EFI) and check it with a meter or by eye. If it's blown and also P0141 or P0155 are set, suspect a shorted heater or wire before simply replacing the fuse.
Tools: Multimeter
Measure the heater resistance
Unplug the sensor and measure across the two heater pins on the sensor side. Most narrowband sensors read somewhere between about 2 and 20 ohms at room temperature; check the spec for your vehicle. An open (infinite) reading confirms a burned-out heater.
Tools: Multimeter
Check power and ground at the harness
With the key on, one heater wire on the harness side should show battery voltage; the other is the PCM-controlled ground. No voltage means a fuse, relay or wiring problem that a new sensor won't fix.
Tools: Multimeter
Inspect the harness routing
Follow the sensor pigtail along its route and look for melted insulation, broken clips or wires resting on the exhaust manifold or heat shield. Repair with high-temperature-rated wire and secure the harness away from heat.
Tools: Flashlight
Replace the sensor properly
Use penetrating oil and an O2 sensor socket; a slightly warm exhaust (not hot) often makes removal easier. Use a direct-fit sensor, don't get anti-seize on the tip, and tighten to spec, typically around 30 lb-ft.
Tools: O2 sensor socket, Torque wrench
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace blown fuse (after finding the cause) | Yes | $1–$10 |
| Replace upstream O2 sensor yourself (part only) | Yes | $50–$180 |
| Replace upstream O2 sensor at a shop | No | $150–$400 |
| Repair melted or broken sensor wiring | No | $80–$250 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Field data
| Chevrolet | 4 | Volt |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
Most OBD-II emissions tests fail a car with the check engine light on, and an incomplete O2 sensor heater monitor can also cause a rejection. After replacing the sensor, drive a few cold-start cycles so the heater and O2 monitors report ready.
Frequently asked questions
Can I just replace the heater instead of the whole O2 sensor?
No. The heater is built into the sensor body and isn't serviceable, so a failed heater means a new sensor. Check the fuse and wiring first so you don't replace a good sensor.
P0135 came back right after I installed a new sensor. Why?
The sensor probably wasn't the problem. A blown fuse, a missing heater voltage from the harness, a melted wire or a poorly spliced universal sensor are the usual culprits. Measure power and ground at the harness connector.
What is the difference between P0135 and P0141?
Both are heater circuit faults, but P0135 is the upstream sensor before the catalytic converter and P0141 is the downstream sensor after it. The upstream sensor controls fueling, so fixing P0135 matters more for mileage.
Is a universal O2 sensor OK for P0135?
It can work, but you have to splice the wires correctly and seal the joints against water and heat. A direct-fit sensor with the factory connector avoids the most common cause of repeat heater codes.