P015C: O2 Sensor Delayed Response - Rich to Lean Bank 2 Sensor 1

What it means

Pre-cat O2 sensor on bank 2 reacts only after delay when mixture switches rich to lean. 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 2 sensor 1 is the upstream oxygen sensor before the catalytic converter on bank 2, and it helps the ECU adjust fuel for that bank. P015C sets when this sensor takes too long to detect a change from rich exhaust to lean exhaust, so the ECU receives that feedback late.

Common causes

Most likely first.

1. Oxygen sensor aged or sluggish
2. Sensor contaminated by oil, coolant, or fuel
3. Exhaust leak near the sensor
4. 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

1. Capture freeze-frame data and watch bank 2 sensor 1 live data during a warm rich-to-lean transition, comparing its timing with bank 1 sensor 1.
2. Inspect the upstream bank 2 sensor connector and wiring for loose or corroded terminals, heat damage, and chafing.
3. Check for an exhaust leak near bank 2 sensor 1 and review bank 2 fuel trims and misfire data for mixture or combustion problems.
4. Look for oil, coolant, or fuel contamination that could have slowed the bank 2 upstream sensor.
5. Test the sensor signal and heater circuit using the vehicle wiring diagram and manufacturer’s specification, then replace the sensor if it remains slow.

Common fixes

• Repair an exhaust leak near bank 2 sensor 1.
• Clean and secure its connector or repair damaged wiring.
• Resolve an engine fluid leak or fueling problem contaminating the sensor.
• Replace bank 2 sensor 1 if it fails rich-to-lean response testing with a sound circuit.

Can I keep driving?

A steady check-engine light usually allows a short, gentle drive, but delayed bank 2 fuel feedback can increase fuel use and emissions. Stop if the light flashes or the engine runs severely rough.