Emissions

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

Severity: medium Safe to drive

What P0031 means

The engine computer controls the heater in the upstream oxygen sensor (Bank 1 Sensor 1) and monitors its circuit. The heater brings the sensor up to operating temperature quickly so the engine can enter closed-loop fuel control sooner. P0031 is set when the computer sees abnormally low voltage or current in the heater control circuit, such as a short to ground or a failed heater element.

Symptoms

  • Check Engine light on
  • Rough or unstable idle, especially on cold starts
  • Worse fuel economy
  • Longer time to reach closed-loop operation
  • Failed emissions test

Possible causes

  1. Failed heater element in the Bank 1 Sensor 1 oxygen sensor
  2. Damaged, chafed or corroded wiring or connector in the heater circuit
  3. Heater control wire shorted to ground
  4. Poor ground connection in the heater circuit
  5. Internal driver fault in the PCM (rare)

How to diagnose

  1. Read codes and freeze-frame data, clear the code and see if it returns
  2. Inspect the sensor harness and connector for chafing, melting or corrosion
  3. Confirm which sensor is Bank 1 Sensor 1 using the engine's wiring diagram
  4. Measure heater resistance at the sensor terminals and compare with spec
  5. Check for power at the heater with the ignition on
  6. Test the control wire for a short to ground with the PCM connector unplugged
  7. Check the heater circuit fuse and ground
  8. Review fuel trims and sensor behavior in live data, and look for exhaust leaks near the sensor

Common fixes

  • Replace the Bank 1 Sensor 1 oxygen sensor with its built-in heater
  • Repair or replace damaged wiring and connectors
  • Restore a clean, tight ground connection
  • Replace or reprogram the PCM only if a driver fault is confirmed

Questions and answers

What does P0031 mean?

It means low voltage in the heater control circuit of the upstream oxygen sensor on bank 1. The sensor itself or its wiring is the usual culprit.

Can I drive with P0031?

Yes, for short trips. Fuel economy will suffer, and prolonged driving with poor mixture control can harm the catalytic converter, so repair it soon.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.