O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)

What it means

The oxygen sensor at Bank 1 Sensor 1 is reacting too slowly to changes in the exhaust. 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). This is the classic sign of an old, worn-out sensor.

Symptoms

• Check engine light on
• Poor fuel economy
• Rough idle or hesitation
• Failed emissions test
• Possible rotten egg smell from a rich mixture

Probable causes

Most likely first

1. 1Aged or contaminated oxygen sensor (most common)
2. 2Exhaust leak near the sensor
3. 3Oil or coolant being burned and coating the sensor
4. 4Wrong fuel or fuel additives contaminating it

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. 2Locate the Bank 1 Sensor 1 sensor (in the exhaust manifold or down-pipe, before the catalytic converter). Inspect the connector and wiring; exhaust heat often melts or chafes these wires.
3. 3Check for exhaust leaks ahead of the sensor (ticking noise, soot marks). Outside air pulled in through a leak makes the sensor read lean.
4. 4Watch the sensor on a graph at 2,500 RPM. A healthy upstream sensor switches from lean to rich several times per second; a lazy sensor switches slowly with rounded peaks.
5. 5Check for oil burning or coolant loss that could be fouling the sensor.
6. 6Check 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
• Fix any exhaust leak
• Repair the source of oil or coolant burning