HO2S Heater Control Circuit Low (Bank 1 Sensor 1)
Code P0031 means the PCM has detected that the heater control circuit for Bank 1 Sensor 1 (upstream O2 sensor) is low — the circuit voltage is below the expected range when the heater is commanded on. This typically indicates a short to ground in the heater wiring or a failed sensor with an internal short. The sensor's oxygen-measuring function may be unaffected, but heating performance is compromised.
What You Might Notice
- Check engine light on
- Delayed closed-loop fuel control on cold starts
- Slightly increased fuel consumption
- May fail emissions test
Most Common Causes
- 1
Short to ground in heater circuit wiring
The heater power wire has shorted to ground, causing the circuit voltage to read low. Inspect the harness for chafing against the exhaust or chassis.
- 2
Failed O2 sensor with internal short
The heater element inside the sensor has shorted internally, pulling the circuit voltage low.
- 3
Corroded or damaged O2 sensor connector
A corroded connector can create a low-resistance path to ground on the heater circuit.
How to Diagnose It
- 1
Check Short to ground in heater circuit wiring — The heater power wire has shorted to ground, causing the circuit voltage to read low. Inspect the harness for chafing against the exhaust or chassis.
- 2
Check Failed O2 sensor with internal short — The heater element inside the sensor has shorted internally, pulling the circuit voltage low.
- 3
Check Corroded or damaged O2 sensor connector — A corroded connector can create a low-resistance path to ground on the heater circuit.
How to Fix It
- 1
Inspect heater circuit wiring for shorts to ground
Trace the Bank 1 Sensor 1 heater wiring for chafing against exhaust components or metal brackets. Repair any damaged insulation.
- 2
Test heater resistance at the sensor
Disconnect the sensor and measure heater terminal resistance. A very low or zero reading indicates an internal short — replace the sensor.
- 3
Replace the upstream O2 sensor if shorted
Install an OEM or OEM-quality replacement sensor.
- 4
Clear codes and verify
Clear DTCs and perform a cold-start drive cycle to confirm heater circuit passes.
Need a deeper diagnosis?
Search for related repair guides or browse by system.