P0135 The heater in the upstream oxygen sensor on bank 1 isn't working as the computer expects

O2 sensor heater circuit, bank 1 sensor 1

Monitor

A failed sensor heater mainly affects emissions after cold starts and the computer's self-tests; the car normally drives the same, so book a repair rather than stopping.

It gets more serious if:

  • Stop: Stop if the check engine light starts flashing: that means an active misfire hot enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.

Source: how we rate severity

P0135 in short

P0135 means the heater inside the upstream oxygen sensor on bank 1 isn't working, so the sensor is slow to warm up. The usual causes are a failed heater element, which means a new sensor, or damaged wiring near the exhaust. The car normally drives fine; book a repair.

Oxygen sensors only give a useful signal once they are very hot. Exhaust heat alone takes a while to get them there, so modern sensors have a small electric heater that brings them up to temperature within seconds of a cold start. Emissions rules require the engine computer to check that heater.

P0135 sets when the heater in the upstream (bank 1 sensor 1, before the catalytic converter) sensor draws the wrong current, shows the wrong resistance, or doesn't warm the sensor in time. On the 2008 Saab 9-3, for example, the computer flags the heater when its resistance falls outside the expected window.

The upstream sensor is the one the computer uses to control fuel. With a dead heater it takes longer to start giving a usable signal after each cold start, so emissions are higher during warm-up, and the computer's oxygen sensor checks may not complete.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Heater element inside the oxygen sensor has failed

    Common general cause
    Why it ranks here
    The heater is built into the sensor and wears out with it, so a failed element is the usual cause. Sensors have a finite life; Bosch puts heated sensors at up to about 100,000 miles.
    What would change this
    Heater resistance outside the specification at the sensor's own connector confirms it; resistance in range moves the wiring and power supply up.
    How to test the oxygen (o2) sensor

  2. Damaged sensor wiring or connector near the hot exhaust

    Common general cause
    Why it ranks here
    The sensor's wiring runs close to the exhaust and under the car, where heat, chafing and corrosion damage it. Saab's service information treats this code as a fault in the leads or the component.
    What would change this
    A melted, chafed or corroded wire or connector, or a fault that comes and goes when the harness is wiggled, confirms it.

  3. No power or ground reaching the heater (fuse, relay or circuit)

    Possible — needs testing
    Why it ranks here
    The heater needs battery voltage and a switched ground. A blown fuse or open feed stops it working even with a good sensor.
    What would change this
    No supply voltage at the sensor connector with the key on confirms it.

  4. Engine computer's heater control circuit has failed

    Possible — needs testing
    Why it ranks here
    The computer switches the heater on and off and checks its own output for opens and shorts. A failed output stage is rare and is diagnosed only after the sensor and wiring test good.
    What would change this
    A good sensor, good wiring and correct power with no control from the computer raises this.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes and note whether other heater or oxygen sensor codes are stored

    Low riskTool: OBD-II scan tool

    • IfP0135 and P0141 are both stored

      It meansBoth heaters failing together points to a shared power feed or fuse.

      Go to step 4

    • IfOnly P0135 is stored

      It meansInspect this sensor's wiring.

      Go to step 2

  2. With the exhaust cold, inspect the upstream (bank 1 sensor 1, before the catalytic converter) sensor's wiring and connector for melting, chafing and corrosion

    Use cautionTool: visual inspection

    Warning:Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

  3. Unplug the sensor and measure heater resistance across the heater terminals on the sensor side, comparing it with your engine's specification

    Use cautionTool: digital multimeter; wiring diagram

    Warning:Let the exhaust cool fully before touching the sensor or its wiring. Back-probe connectors gently, never pierce insulation, and follow the wiring diagram for your engine.

  4. With the key on, check heater supply voltage at the harness connector, then the computer's control (ground) side

    Best left to a shopTool: digital multimeter; wiring diagram

    Warning:Let the exhaust cool fully before touching the sensor or its wiring. Back-probe connectors gently, never pierce insulation, and follow the wiring diagram for your engine.

Symptoms

  • Check engine light with no change in how the car drives
  • Oxygen sensor readiness monitor not completing, causing a failed or refused inspection
  • Slightly worse fuel economy after cold starts

How to fix

  1. With the exhaust fully cold, follow the sensor's wiring to its connector and check for melted insulation, chafing on the exhaust or body, and a loose or corroded connector.

  2. Check the fuse for the oxygen sensor heaters if your owner's manual lists one, especially if both heater codes are stored.

Parts and cost context

  • Oxygen (O2) sensor:The heater is part of the sensor, so a failed heater means a new sensor. Use the correct sensor for that position and engine; upstream and downstream sensors are often different parts.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

We don't have reviewed vehicle-specific evidence for P0135 yet. The causes and tests above apply across vehicles.

  • P0141The heater in the downstream oxygen sensor on bank 1 isn't working as the computer expects

    Same system Possible — needs testing

    P0141 is the heater circuit for the downstream sensor on the same bank. Both heaters are monitored the same way; if both codes appear together, check a shared fuse or power feed first.

  • P0171Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

    Same system Possible — needs testing

    The upstream sensor feeds fuel trim. A heater fault alone doesn't set a lean code, but if both are present, repair the heater first so the trim readings can be trusted.

All Oxygen sensors codes

P0135 questions

Can I drive with P0135?

Yes, in most cases. The car usually drives normally, but emissions after cold starts are higher and the oxygen sensor monitor can't complete, so book a repair. If the check engine light ever flashes, stop driving it.

Which oxygen sensor is bank 1 sensor 1, and which is sensor 2?

Bank 1 is the side of the engine with cylinder 1; an inline engine has only bank 1. Sensor 1 is the upstream sensor before the catalytic converter; sensor 2 is the downstream sensor after it. Check a diagram for your engine before buying a part.

Do I have to replace the whole sensor?

If the heater element itself has failed, yes; it can't be repaired separately. Check the wiring, connector and fuse first, because those are cheaper fixes that set the same code.

Will my car pass an emissions inspection with this code?

Not while the check engine light is on; a lit light with stored codes fails the federal OBD inspection. After the repair the light normally goes out after three good drives, and the oxygen sensor and catalyst monitors must complete before a retest.

Sources and review

  1. Bosch Oxygen Sensors trade brochure — Robert Bosch LLC ,
  2. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  3. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  4. Saab 9-3 (2008) WIS DTC P0135 00 (ME9): O2S 1 heater resistance out of range — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  5. Saab 9-3 (2008) WIS DTC P0135 04 (Simtec): O2 sensor heater circuit open (bank 1 sensor 1) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  6. Saab 9-3 (2008) WIS DTC P0141 00 (ME9): O2S 2 heater resistance out of range — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  7. P0171 Code: Causes, Symptoms, and How to Fix It — Identifix ,
  8. Bosch Oxygen Sensors consumer brochure — Robert Bosch LLC ,
  9. A/F Sensors — DENSO Auto Parts
  10. 40 CFR §85.2222 Onboard diagnostic test procedures — U.S. Environmental Protection Agency (Electronic Code of Federal Regulations)
Last reviewed
Written by
Saurabh K.
Editorial review
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