What it means
Pre-cat O2 sensor on bank 1 reacts only after delay when mixture switches lean to rich. A drifted or sluggish lambda sensor lets the mixture run lean or rich without correction — fuel economy and emissions suffer immediately.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 sensor 1 is the upstream oxygen sensor before the catalytic converter on bank 1, and it helps the ECU correct that bank’s fuel mixture. P015B sets when the sensor is too slow to register a change from lean exhaust to rich exhaust, delaying fuel correction in that direction.
Common causes
Most likely first.
- Oxygen sensor aged or sluggish
- Sensor contaminated by oil, coolant, or fuel
- Exhaust leak near the sensor
- Sensor wiring or connector fault
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Record freeze-frame data and use warm live data to observe a lean-to-rich transition at bank 1 sensor 1, comparing its response with bank 2 sensor 1.
- Inspect the upstream bank 1 sensor connector and wiring for corrosion, loose pins, heat damage, or rubbing against nearby parts.
- Check for an exhaust leak near bank 1 sensor 1 and review bank 1 fuel trims and misfire data for mixture or combustion faults.
- Check for oil, coolant, or fuel contamination that could make the upstream sensor sluggish.
- Test the sensor signal and heater circuit to the manufacturer’s specification, then replace bank 1 sensor 1 if its lean-to-rich response remains slow.
Common fixes
- Repair an exhaust leak near bank 1 sensor 1.
- Restore a secure connector or repair damaged upstream sensor wiring.
- Fix an engine leak or fueling fault that is contaminating the sensor.
- Replace bank 1 sensor 1 if it fails response testing with a sound circuit.
Can I keep driving?
A short, gentle drive is usually possible with a steady light, but delayed rich detection can impair fuel control and increase emissions. Avoid hard acceleration and stop if the light flashes or the engine runs badly.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.