P0031 — Oxygen sensor heater control circuit low — bank 1, sensor 1

P0031 indicates an abnormally low electrical level in the heater control circuit for bank 1, sensor 1. This is the upstream sensor used for mixture control. “Low” describes the electrical circuit, not a lean mixture. Supply wiring and heater control strategies differ between vehicles.

Affected system: Powertrain

Symptoms

• Engine warning lamp and a stored code.
• Delayed sensor readiness visible in diagnostic data after starting.
• Fuel consumption may increase if mixture control is affected.

Possible causes

• Control wire shorted to ground or damaged contacts.
• Heater or supply fault; interpretation depends on the driver circuit.

What to check

• Confirm B1S1 in the engine diagram and save related codes and freeze-frame data before clearing anything.
• With the exhaust cold, inspect the B1S1 harness for melted insulation, rubbing, water in the connector and loose terminals.
• Using the OEM diagram, distinguish supply from control wiring; check for a ground short without assuming the control polarity.
• Test element resistance and control response separately using suitable equipment; do not apply voltage directly to unidentified pins.
• After correcting the confirmed fault, repeat the OEM monitor conditions and check for code recurrence, beyond merely extinguishing the lamp.

Also written as: P0031-00, P003100, PO031 (some scan tools add the fault-type byte, e.g. -00).