P0031 The heater control circuit for the upstream oxygen sensor on bank 1 is reading too low

O2 sensor heater control circuit low, bank 1 sensor 1

Monitor

A heater circuit fault 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

P0031 in short

P0031 means the engine computer sees a voltage that is too low in the heater circuit of the upstream oxygen sensor on bank 1. The usual causes are wiring shorted to ground or a bad connector near the exhaust, a failed heater inside the sensor, or a lost power supply. The car normally drives fine; book a repair.

Oxygen sensors only give a useful reading once they are very hot, so each has a small electric heater that the engine computer switches on after a cold start. The computer also watches the voltage on that heater circuit to make sure the switching works.

P0031 sets when the voltage on the upstream (bank 1 sensor 1, before the catalytic converter) sensor's heater circuit stays close to zero when it shouldn't. On the 2008 Saab 9-3, for example, it sets after a few seconds of near-zero voltage while the engine is running. That usually means a wire shorted to ground, a broken feed or a poor connection, and sometimes the heater itself.

This is the sensor the computer uses to control fuel, so with no heater it takes longer after each cold start to begin fine-tuning the mixture.

Most likely causes

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

  1. Heater wiring shorted to ground, broken, or a poor connector contact

    Common general cause
    Why it ranks here
    The heater control wire runs along the hot exhaust, where heat and chafing damage insulation. Saab's service information treats this code as an electrical fault in the leads or the sensor, and starts by checking the harness and connectors.
    What would change this
    Melted or chafed insulation, a wire touching the body, or a fault that comes and goes when the harness is wiggled confirms it. Oxidised or pushed-back connector pins count too.

  2. Failed heater element inside the oxygen sensor

    Common general cause
    Why it ranks here
    The heater is built into the sensor and can fail with it. Saab's 9-2X procedure replaces the sensor when its heater resistance is out of range and the wiring is good.
    What would change this
    Heater resistance out of specification at the sensor's own connector confirms it; a reading in range moves the wiring and supply up.
    How to test the oxygen (o2) sensor

  3. No power reaching the heater from the main relay or fuse

    Common general cause
    Why it ranks here
    The heater gets battery voltage through the main relay. Saab's 9-2X procedure checks for more than 10 volts at the sensor connector and, when it is missing, repairs the feed or connector.
    What would change this
    No supply voltage at the connector with the key on confirms it. Both front and rear heater codes stored together point here first.

  4. Poor ground for the engine computer

    Possible — needs testing
    Why it ranks here
    The computer switches the heater through its own ground circuit. Saab's 9-2X procedure measures the computer's ground resistance before going further.
    What would change this
    Ground resistance above the specified limit, or a loose or corroded ground terminal, raises this.

  5. Engine computer's heater control output has failed

    Possible — needs testing
    Why it ranks here
    A failed output inside the computer is rare. Saab's 9-2X rear-heater procedure only reaches computer replacement after the wiring, supply and sensor test good, and first repairs any short in the harness.
    What would change this
    A good sensor, good wiring and correct supply, with an output voltage that is still wrong, raises this. A software update from the vehicle maker may apply first.

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 the other heater circuit code is stored as well

    Low riskTool: OBD-II scan tool

    • IfP0031 and P0037 are both stored

      It meansBoth heaters losing power together points to their shared supply.

      Go to step 4

    • IfOnly P0031 is stored

      It meansInspect this sensor's wiring first.

      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. Then, with the engine idling, gently move the harness and watch whether the code or heater reading changes

    Use cautionTool: visual inspection; scan tool

    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. With the ignition off, unplug the sensor and measure heater resistance across its heater terminals, 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. Key on, check heater supply voltage at the harness connector; then check the computer's ground resistance and its heater output

    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
  • Both P0031 and P0037 stored together

How to fix

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

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

Parts and cost context

  • Oxygen (O2) sensor:Replace the sensor only if its heater measures open or out of specification; wiring and supply faults set the same code. 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

Ram

  • 1812222Status unverified

    PCM software update for P0031

    2022promaster · issued · NHTSA 10221510

    For a lit MIL with P0031 (upstream O2 heater circuit low), the PCM is reprogrammed with the latest software.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0037The heater control circuit for the downstream oxygen sensor on bank 1 is reading too low

    Same system Common general cause

    P0037 is the same heater circuit fault on the downstream sensor. Saab's 9-2X procedure says that when both are stored, check the shared power supply from the main relay first.

  • P0135The heater in the upstream oxygen sensor on bank 1 isn't working as the computer expects

    Same system Possible — needs testing

    P0135 also reports a problem with the upstream sensor's heater, judged on how it performs rather than on the circuit voltage. The same wiring and sensor checks apply to both.

All Oxygen sensors codes

P0031 questions

Can I drive with P0031?

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.

What is the difference between P0031 and P0135?

Both concern the same sensor's heater. P0031 means the computer saw the wrong voltage on the heater circuit, which usually points at wiring, power or a short. P0135 means the heater itself is not behaving as expected; on the 2008 Saab 9-3, for example, it sets when the heater resistance is out of range. The wiring and sensor checks are the same for both.

Do I have to replace the oxygen sensor?

Not always. A shorted or broken wire, a corroded connector or a lost power feed sets this code too, so those are checked before the sensor. If the heater inside the sensor measures open, the sensor must be replaced.

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. Saab 9-3 (2008) WIS DTC P0031 00 (ME9): O2S 1 preheater circuit, short to ground — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  2. Bosch Oxygen Sensors trade brochure — Robert Bosch LLC ,
  3. 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
  4. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  5. Saab 9-2X 2.5i (2005) WIS DTC P0031: HO2S heater control circuit low (bank 1 sensor 1) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  6. Saab 9-2X 2.5i (2005) WIS DTC P0037: HO2S heater control circuit low (bank 1 sensor 2) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  7. 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)
  8. FCA US (Stellantis) bulletin 1812222: PCM software update for P0031 — FCA US (Stellantis) (filed with NHTSA) ,
  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.
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