P0135 Front O2 Sensor Heater Fault (Bank 1)
Get it checked soon
The small heater inside the bank 1 front (upstream) oxygen sensor has an electrical fault. Usually the sensor itself has failed and needs replacing. Drivable, but fueling after a cold start is worse until it is fixed.
- Safe to drive?
- Yes, fix soon
- Typical cost at a shop
- $125-$523 RepairPal national averages, parts and labor
- Emissions test
- Yes, while the check engine light is on, most OBD emissions tests fail the car.
Watch the oxygen sensor after a cold start to see if its heater, a fuse or the wiring is at fault.
$34.99 preorder, regular price $49. Free shipping in the US.
Codes and live sensors are free in the app. Obie, the AI mechanic, is part of ScanMyCar Pro.
What it means
Oxygen sensors only work when they are very hot, so each one has a small electric heater inside. P0135 is about the front (upstream) oxygen sensor, before the catalytic converter on bank 1. The computer saw that its heater circuit draws the wrong current or none at all. It steers the fuel mixture, so until it is hot the computer runs on fixed fueling.
The technical version
The computer powers the heater and monitors its current or circuit voltage. An open heater element, a short, a blown fuse or high resistance in the wiring sets P0135. The heater and the sensor are one part, so a failed heater means a new sensor. Bank 1 is the side of the engine with cylinder 1; on an inline engine there is only bank 1.
Official name: O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1).
Symptoms
- Worse fuel economy, mostly on short trips.
- Sometimes rough running when cold.
- Often only the light.
Causes, most likely first
| Cause | How likely | How to tell |
|---|---|---|
| A failed heater element inside the sensor | most common | The heater measures open with a meter. |
| A blown fuse for the oxygen sensor heaters | common | Other heater codes set at the same time. |
| Damaged or corroded wiring or connector | common | Melted insulation near the exhaust, or green corrosion in the plug. |
| An engine computer fault | rare | The sensor and wiring test good. |
How likely is our ranking from repair data and common practice, not a published rate.
What the live data shows
After a cold start, a healthy front sensor starts switching within a minute or two and the fuel system goes to closed loop. With P0135, closed loop comes late or not at all, and the sensor reading stays flat for longer. Some cars also report the heater current or status.
What to check first, in order
-
Check the fuse for the oxygen sensor heaters.
A cheap check.
What the data shows: No reading needed.
Cost: free
-
Inspect the sensor wiring and connector.
Heat and road salt damage them.
What the data shows: No reading needed.
Cost: free to look
-
Have the heater resistance and its power and ground checked.
It tells a failed sensor from a wiring fault.
What the data shows: Not a scanner step.
Cost: $125-$183 at a shop (Check engine light diagnosis (shop), RepairPal)
-
Replace the front oxygen sensor.
The heater is part of the sensor.
What the data shows: The code stays away and the sensor warms up quickly.
Cost: $440-$523 at a shop (Oxygen sensor replacement, RepairPal)
Is it safe to drive?
Get it checked soon
Yes, fix soon. The engine runs on fixed fueling for longer after each cold start.
Drivable. After each cold start the engine waits longer before it uses the sensor, so fuel economy and emissions suffer, and on some cars the computer stays on fixed fueling. Get it fixed soon.
Stop driving and get help if
- The check engine light flashes.
- The engine misfires or runs very rough.
What it costs
| Repair | Typical cost |
|---|---|
| Check engine light diagnosis (shop)RepairPal, updated September 3, 2026 | $125-$183 |
| Oxygen sensor replacementRepairPal, updated September 3, 2026 | $440-$523 |
National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. Oxygen sensor replacement was CarMD's number 2 check engine repair in 2025, at an average of $287. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates.
Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026).
Can I clear it? Will it come back?
The heater test runs on every drive, so a fault still there usually sets the code again within one or two drives. After the repair, clear it and drive; the oxygen sensor monitor should complete within a few drives.
Will it fail an emissions test?
Yes, while the check engine light is on, most OBD emissions tests fail the car.
Related codes
Also related, no guide yet: P0133, P0155.
How Obie handles P0135
Obie reads the oxygen sensor data and fuel status after a cold start and tells you whether the pattern points to the heater, a fuse or the wiring. Obie is part of ScanMyCar Pro. Codes and live sensors are free.
Questions people ask
Is P0135 the sensor or the wiring?
Usually the sensor, because the heater is inside it. Check the fuse and the connector first.
Can I drive with P0135?
Yes. Get it fixed soon: fuel economy and emissions suffer after every cold start.
Which sensor is bank 1 sensor 1?
Bank 1 is the side of the engine with cylinder 1. Sensor 1 is the one before the catalytic converter.
Sources
- RepairPal: P0135 code page repairpal.com
- RepairPal: Check engine light diagnosis (shop) (updated September 3, 2026) repairpal.com
- RepairPal: Oxygen sensor replacement (updated September 3, 2026) repairpal.com
- CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
- Texas DPS emissions testing FAQ dps.texas.gov
- Arizona myazcar.com: OBD and the check engine light myazcar.com
A guide, not a diagnosis. This page describes what P0135 generally means across OBD2 cars. What is actually wrong with your car depends on its live data and its history, and any repair decision deserves a proper diagnosis, whether that is you with a scanner or a professional with a lift.