O2 Sensor Heater Circuit (Bank 1 Sensor 1)

What it means

The heater inside the oxygen sensor at Bank 1 Sensor 1 is not working properly. This is the upstream oxygen sensor on bank 1, mounted before the catalytic converter. It measures oxygen in the exhaust so the ECU can keep the fuel mixture correct. Bank 1 is the side of the engine with cylinder 1 (on 4-cylinder engines there is only bank 1). The heater gets the sensor to operating temperature quickly after a cold start; without it, the ECU stays in open-loop fuel control longer and emissions go up.

Symptoms

• Check engine light on
• Slightly worse fuel economy after cold starts
• Usually no other symptoms

Probable causes

Most likely first

1. 1Heater element burned out inside the sensor (most common)
2. 2Blown fuse for the O2 heater circuit
3. 3Open or damaged heater wiring
4. 4Heater relay fault

How to diagnose it

Work through these in order before buying parts

1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
2. 2Check the O2 sensor heater fuse.
3. 3Unplug the sensor and measure resistance across the two heater pins (usually the two same-coloured wires). Most heaters read about 2–20 Ω; an open circuit means the sensor is bad.
4. 4With the key on, check for battery voltage on the heater supply wire at the connector.
5. 5Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

• Replace the oxygen sensor (the heater is built in)
• Replace a blown fuse and find out why it blew
• Repair the heater wiring