P0031: OBD-II code P0031 flags a low-voltage condition on the upstream bank 1 O2 sensor heater circuit — usually a shorted-to-ground drive wire or a shorted sensor heater.

Symptoms

• Check engine light is on; the sensor can't reach operating temperature, so closed-loop control is delayed after start-up.
• Extended rich open-loop running after cold start, reducing fuel economy on short trips.
• May blow the heater fuse repeatedly if the short is severe enough to overload the circuit.

Common causes

• A shorted-to-ground heater control wire between the PCM and the sensor connector.
• An internally shorted O2 sensor heater element pulling the drive line low.
• A failed PCM heater driver stuck in the low state — rare but possible.

What to do?

Disconnect the sensor and check the heater drive wire for continuity to ground with the connector unplugged — there should be no continuity. Measure sensor heater resistance (should read 8-15 Ω). Replace the sensor or repair the wiring. Check the PCM heater fuse for damage.