P015D: O2 Sensor Delayed Response - Lean to Rich Bank 2 Sensor 1

What it means

Pre-cat O2 sensor on bank 2 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 2 sensor 1 is the upstream oxygen sensor before the bank 2 catalytic converter, and its readings help the ECU adjust fuel delivery. P015D sets when the sensor responds too slowly to a change from lean exhaust to rich exhaust, delaying the ECU’s response to that transition.

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. Save freeze-frame data and observe bank 2 sensor 1 during a warm lean-to-rich transition, comparing the response with bank 1 sensor 1.
2. Inspect the upstream bank 2 sensor connector and wiring for corrosion, loose pins, heat damage, or 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 faults.
4. Look for oil, coolant, or fuel contamination that may have made the upstream bank 2 sensor sluggish.
5. Test the signal and heater circuits to the manufacturer’s specification, then replace bank 2 sensor 1 if its lean-to-rich response remains slow.

Common fixes

• Repair an exhaust leak near bank 2 sensor 1.
• Secure the connector or repair damaged upstream sensor wiring.
• Fix an engine leak or fueling fault that is contaminating the sensor.
• Replace bank 2 sensor 1 if it fails response testing after circuit checks.

Can I keep driving?

With a steady light, a short careful drive is generally possible, but delayed bank 2 feedback can worsen fuel economy and emissions. Avoid hard driving and stop if the engine runs poorly or the light flashes.