GarageGuideOxygen sensor heater (Bank 1, Sensor 1)
P0135 Code: O2 Sensor Heater Circuit (Bank 1 Sensor 1)
P0135 stands for "O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)." Sensor 1 is the upstream oxygen sensor, the one before the catalytic converter, and Bank 1 is the side of the engine with cylinder 1. On a four-cylinder engine there is only one bank, so it is simply the front oxygen sensor.
Oxygen sensors only work when they are very hot. To get them working quickly after a cold start, each sensor has a small electric heater built in. P0135 means the computer sent power to that heater and did not see it working. The sensor may still read oxygen once the exhaust heats it up, which is why the car usually drives normally.
P0135 is an emissions-related code. It does not damage the engine and the car normally drives fine. The main downsides are slightly worse fuel economy, higher emissions right after start-up, and a failed emissions test.
You can keep driving normally. Fix it when convenient, especially before an emissions inspection. The check engine light being on does mean you would not notice if a more serious problem set a new code, so do not leave it for months.
Symptoms you might notice
- Check engine light on
- Usually no change in how the car drives
- Sometimes slightly worse fuel economy, especially on short trips
- Possible rough running for the first minute or two after a cold start
- A failed emissions test
Common causes
Heater element failed inside the sensor
This is by far the most common cause. The heater burns out like a light bulb filament, and because it is built into the sensor, the whole sensor is replaced.
Blown fuse for the O2 sensor heaters
Many cars share one fuse among the oxygen sensor heaters. If it blows, you may see P0135 along with other heater codes like P0141.
Damaged wiring or connector
Oxygen sensor wires run close to the hot exhaust. They can melt, chafe, or corrode, cutting power to the heater.
Corroded connector pins
Water and road salt can corrode the connector, adding resistance so the heater cannot draw enough current.
Engine computer heater driver fault
Rarely, the circuit inside the engine computer that powers the heater fails. This is usually only considered after the sensor, fuse, and wiring are confirmed good.
Before you buy parts
P0135 is about the heater inside the sensor, not whether the sensor reads oxygen correctly. In most cases the fix is a new upstream oxygen sensor, but check the fuse and wiring first. A blown fuse can trigger heater codes for several sensors at once, and replacing two sensors when a fuse was the problem is an expensive mistake.
What to check
- Read all codes. If several O2 heater codes appear at once (like P0135 and P0141), check the shared fuse first, because two sensors rarely fail on the same day.
- Check the fuse for the O2 sensor heaters. Your owner's manual or the fuse box lid shows which one.
- Follow the upstream sensor's wire to its connector and look for melted insulation, chafing against the exhaust, or green corrosion on the pins.
- With the connector unplugged, measure the heater resistance across the heater terminals with a multimeter and compare to the specification for your sensor. An open circuit (no reading at all) means the heater is burned out.
- With the key on, check that the connector is receiving voltage on the heater power wire. No power points to a fuse or wiring problem, not the sensor.
Possible repairs
Replace the upstream oxygen sensor
If the heater tests open or out of spec, replace the sensor. An oxygen sensor socket makes the job much easier, and soaking the threads in penetrating oil helps on older cars.
Replace the blown fuse
If the fuse is blown, replace it, then find out why. A fuse that blows again points to a shorted heater or wire.
Repair the wiring
Melted or chafed wires can be repaired and re-secured away from the exhaust so they do not fail again.
Clean or replace the connector
Corroded pins can sometimes be cleaned. Badly corroded connectors are replaced with a pigtail repair.
Parts and tools that may help
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Frequently asked questions
Is P0135 serious?
No. It is an emissions-related code that usually does not affect how the car drives. It can slightly hurt fuel economy and will fail an emissions test, so fix it when convenient.
Can I drive with P0135?
Yes. The car is safe to drive. Plan to fix it before your next inspection, and so that a more serious new problem would not be hidden behind the light that is already on.
Do I need to replace the O2 sensor for P0135?
Usually, yes, because the heater is built into the sensor. But check the fuse and wiring first, especially if more than one heater code is set.
Which O2 sensor is Bank 1 Sensor 1?
It is the upstream sensor, before the catalytic converter, on the side of the engine with cylinder 1. On four-cylinder engines it is simply the front oxygen sensor.
Will clearing P0135 fix it?
No. Clearing the code only turns the light off temporarily. If the heater is still not working, the code will come back, usually within a drive or two.