P0031HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The upstream oxygen sensor on bank 1 (sensor 1, before the catalytic converter) only delivers a usable signal once its ceramic element is hot. A heating element inside the sensor gets it there quickly after a cold start. Typically the heater receives battery voltage via a fuse or relay, and the engine control module switches the ground side with a pulsed signal while comparing the commanded state with the measured voltage or current. P0031 means the control module measures too little voltage or current while the heater is switched on. Depending on the system this points to a burnt-out element, a missing supply or an interrupted control wire. The sensor then warms up late, so mixture control starts late and cold-start emissions rise.
- What you may notice
- Check engine light on
- Closed-loop control starts late after a cold start
- Further heater codes if a shared fuse has blown
- Emissions test or readiness check failed
- Likely causes
- LikelyHeating element burnt out or high resistance (too little current)
- LikelySupply wire interrupted or fuse blown
- PossibleControl wire to the engine control module interrupted
- PossibleCorroded connector with high contact resistance
- RareOutput driver in the engine control module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €80 – €350
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.