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P0031 Generic (SAE)

HO2S Heater Control Circuit Low (Bank 1 Sensor 1)

Updated 2026-10-02 · Fuel, air metering & auxiliary emission controls

Severity
Medium
Typical repair cost
10 – 350 EUR
System
Powertrain

What it means

The ECU switches the heater of the upstream bank 1 oxygen sensor on and off via a low-side driver and reads back the voltage on that wire. P0031 sets when the voltage stays low (near 0 V) at a time the driver is off and should see battery voltage through the heater. That means either no supply reaches the heater (fuse, relay, open heater element) or the control wire is shorted to ground.

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Symptoms

  • Check Engine light with no obvious drivability change
  • Longer warm-up period before fuel trims start to work (open loop)
  • Slightly increased fuel consumption and higher emissions
  • Hesitation or uneven idle in the first minutes after a cold start
  • Often stored together with P0135 or heater codes for other sensors when the fuse is blown

Common causes

  1. Blown fuse for the O2 sensor heaters (shared fuse, so other heater codes often appear at the same time)
  2. Open heater element inside the bank 1 sensor 1 oxygen sensor
  3. Heater control wire chafed through and shorted to ground, typically where it crosses the exhaust manifold or gearbox
  4. Corroded or spread terminals in the sensor connector interrupting the supply
  5. Failed O2 heater relay or main relay contact
  6. Shorted heater driver transistor in the ECU

How to diagnose

  1. Check the O2 heater fuse first (engine bay fuse box, typically 15-20 A). If it is blown, do not simply replace it: a shorted heater or harness will blow it again, so measure each sensor's heater for a short to its signal pins or housing.
  2. Unplug bank 1 sensor 1 and measure heater resistance across its two heater pins: 2-15 ohm at 20 °C is normal; open circuit confirms a failed sensor. Also check resistance from each heater pin to the sensor body: should be infinite.
  3. Key on, measure supply voltage on the harness side of the connector: 12-14.5 V required. 0 V with a good fuse means an open supply wire or a bad relay.
  4. Measure the control pin on the harness side with the sensor unplugged, key on: expect battery voltage or around 5-12 V pulled up by the ECU. 0 V indicates the control wire is shorted to ground or the ECU driver is shorted; disconnect the ECU connector and measure wire to ground (should be over 10 kohm).
  5. Reconnect the sensor and measure heater current at idle: 0.8-2 A (up to 3-4 A for wideband). Zero current with good supply points to an open heater or an ECU driver that never switches.
  6. Wiggle-test the harness while watching live heater status on the scanner; repair chafed or melted sections and secure the harness away from the exhaust.

Frequently asked questions

P0031 and P0037 appeared together after a car wash. Why?

Both heaters usually share one fuse and supply relay. Water in a connector or a shorted wire blew the fuse, so both sensors lost heater power. Find and dry the connector, then replace the fuse.

Is P0031 the same as P0135?

No. P0135 is the heater performance code (the heater does not warm the sensor fast enough); P0031 is an electrical circuit fault with voltage reading low. P0031 is almost always wiring, fuse or an open heater.

Related codes

P0030P0032P0135P0051P0037

Nearby codes in this group