P0031: HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
The control circuit for the upstream bank 1 oxygen sensor heater is reading low, often from a short to ground or a failed heater.
What P0031 means
P0031 is a more specific version of P0030. It means the computer sees the heater control circuit for the bank 1 upstream oxygen sensor at a lower voltage or current than expected. Depending on how the maker wires it, that can mean a heater element that has shorted, a control wire shorted to ground, or a blown fuse that removes power from the heater. RepairPal lists oxygen sensor failure, wiring issues and engine computer failure as causes, and sensor replacement as the common fix. Without the heater, the upstream sensor takes longer to reach working temperature, so the computer runs on preset fuel tables longer after each cold start. That wastes fuel and raises emissions. Shops check the fuse, measure heater resistance and test for a short to ground in the harness. If the fuse blows again after replacement, there is a hard short in the wiring or sensor that must be found first.
Common symptoms
- Check engine light on
- Worse gas mileage
- Slightly reduced performance on some vehicles
- Longer time before the engine enters closed loop
- Failed emissions test
Most common causes
- Oxygen sensor heater shorted internally
- Heater control wire shorted to ground
- Blown fuse in the heater power circuit
- Damaged connector
- Engine computer fault (rare)
What to check yourself first
Check the fuse that powers the oxygen sensor heaters. Inspect wiring near the exhaust manifold for melted insulation touching metal. Measure heater resistance at the sensor.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | RepairPal lists oxygen sensor replacement as the usual repair. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a shorted or chafed wire. |
| Check engine light diagnosis at a shop | $125-$183 | Confirms the fault before replacing parts. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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