P0031: HO2S Heater Control Circuit Low (Bank 1 Sensor 1)

P0031 means the engine computer measured too little current or too low a voltage in the heater circuit of the upstream oxygen sensor on bank 1, the one before the catalytic converter. On many cars that is exactly what a burned-out heater, a blown fuse or a broken supply wire produces. Until exhaust heat warms the sensor, the engine runs on a fixed, richer mixture. A new upstream sensor typically costs about 100 to 300 EUR fitted.

P0031 is a emissions-related diagnostic trouble code classified as low severity. When your vehicle's ECU detects this condition, it stores P0031 and illuminates the check engine light.

If your vehicle has triggered P0031, you may notice one or more of these symptoms:

• Check engine light on, usually with no change in how the car drives once warm
• Slightly higher fuel use on short trips and in cold weather
• Engine stays in open loop (fixed, richer mixture) longer after a cold start
• Code returns within seconds or minutes of starting the engine after being cleared
• Possible rejection at an emissions inspection while the warning light is on

Here are the most common causes of P0031:

Generally yes, for short-term driving. Safe to drive in the short term. The upstream sensor starts working once exhaust heat warms it, so the car drives normally when warm; you lose some fuel economy on short trips and cold-start emissions rise. Book the repair soon, because the warning light hides any new fault and can get the car rejected at an emissions inspection.

Follow these steps to pinpoint the root cause of P0031:

1. Read all stored codes and freeze frame. P0031 together with P0037 or other heater codes points to a shared fuse, relay or supply wire rather than two failed sensors
2. Check the heater fuse named in the owner's manual or on the fuse box lid (often labelled for the O2 or A/F sensor heater, or shared with the engine management). Replace it if blown, and if it blows again look for a short
3. Unplug the Bank 1 Sensor 1 connector and measure the resistance across the two heater pins. Wideband air-fuel ratio sensors often read around 2 ohms and many narrowband sensors between about 3 and 15 ohms; check your car's specification. An open circuit means a burned-out heater
4. With the key on, check for battery voltage on the heater supply pin on the harness side. No voltage points to the fuse, relay or supply wire
5. Check the control wire from the sensor to the engine computer for breaks and for a short to ground, and inspect the connector for corrosion or pushed-back pins
6. After the repair, clear the code and restart. On many cars P0031 is checked soon after start-up, so a genuine fix should keep it from returning. With a generic OBD2 reader, confirm the fuel system goes into closed loop within a few minutes and the oxygen sensor heater readiness monitor completes

Repair costs for P0031 vary depending on the root cause and your vehicle. Here are typical estimates:

P0031 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

These codes are often seen alongside P0031 or indicate related issues: