OBD-II Code P0031 — What It Means & How to Fix It

What Does This Error Mean?

P0031 means the heater circuit inside the Bank 1 Sensor 1 oxygen sensor has low voltage. Oxygen sensors have a built-in heater that warms them up quickly after a cold start. When that heater circuit draws less current than expected, this code is set. The most common causes are a blown fuse, a bad oxygen sensor, or a wiring problem. You may not notice any drivability symptoms, but fuel economy may suffer.

Affected Models

All vehicles 1996+
Common in Toyota Camry
Common in Honda Civic
Common in Chevrolet Silverado
Common in Ford Focus

Common Causes

Failed oxygen sensor heater element inside the sensor itself
Blown fuse for the oxygen sensor heater circuit
Open or shorted wire in the heater circuit between the ECM and the sensor
Corroded or damaged connector at the oxygen sensor
Faulty ECM output for the heater control circuit (rare)

How to Fix It

Check the fuse box for a blown fuse related to the oxygen sensor heater or the upstream O2 sensor circuit. Your owner's manual or the fuse box lid will show the fuse location. Replace any blown fuse with the correct amperage.

If the new fuse blows again immediately, there is a short circuit in the wiring — do not keep replacing fuses. Have a mechanic trace the short.
Locate the Bank 1 Sensor 1 oxygen sensor. Bank 1 is the side of the engine containing cylinder 1. Sensor 1 is before (upstream of) the catalytic converter. Inspect the sensor's wiring connector for corrosion or damaged wires.

On most four-cylinder engines, the upstream O2 sensor is bolted into the exhaust manifold or the pipe just after it. It has a wiring pigtail with typically 4 wires.
Use a multimeter to test the heater circuit resistance across the heater terminals of the sensor. The heater wires are usually the two matching-colored wires (often white). Most heaters read 2-20 ohms — an open reading means the heater has burned out.

If you are not sure which terminals are the heater wires, look up the wiring diagram for your specific vehicle online.
If the sensor fails the resistance test, replace it. Use a quality replacement oxygen sensor — OEM or a reputable brand like Denso, Bosch, or NTK. Cheap universal sensors can cause repeat codes.

Always use anti-seize compound on the sensor threads if they are not pre-coated. This prevents the sensor from seizing into the exhaust and being difficult to remove later.
Clear the code after the repair and drive until the engine reaches full operating temperature. The heater circuit activates within seconds of starting. If the code does not return after 1-2 drive cycles, the repair is complete.

If P0031 returns after a new sensor, the problem is in the wiring harness between the sensor and the ECM — a mechanic will need to trace the circuit.

When to Call a Professional

P0031 is usually a straightforward DIY or inexpensive shop repair. An oxygen sensor replacement typically costs $100-$250 parts and labor at a shop. If the fuse keeps blowing, a mechanic needs to trace a short in the wiring. Do not ignore this code — a cold oxygen sensor runs rich until it warms up, wasting fuel. Prolonged rich running can also damage the catalytic converter over time.

Frequently Asked Questions

Why does an oxygen sensor need a heater?

Oxygen sensors only work correctly above about 600°F (315°C). On a cold engine, it takes several minutes for exhaust heat to warm the sensor up naturally. The built-in heater gets the sensor to working temperature within 30 seconds of startup. This allows the engine computer to enter closed-loop fuel control faster. Faster closed-loop means better fuel economy and fewer emissions right after a cold start.

Will P0031 affect my gas mileage?

Yes, potentially. A cold oxygen sensor cannot accurately measure exhaust gases. The engine runs richer (more fuel) until the sensor warms up. With a broken heater, warmup takes much longer or never happens fully. You might see a small decrease in fuel economy — typically 1-3 miles per gallon.

Is it safe to drive with P0031?

It is generally safe to drive short term. The engine will still run, and you will not be stranded. However, running rich for extended periods wastes fuel and can damage the catalytic converter. Fix this code within a few weeks to protect your exhaust system and maximize fuel economy.