P015D: O2 Sensor Delayed Response - Lean to Rich Bank 2 Sensor 1
What to do
Ask for an exhaust leak check close to the sensor. Going rich means oxygen disappearing from the exhaust, so air leaking in near the sensor masks that change and delays it — and a leak is far cheaper to fix than a sensor is to replace.
Ask whether a heater fault is stored for the same sensor. These sensors have a built-in heater, and one that is failing leaves the sensor too cool to respond properly — in which case that is the real fault.
Ask whether the sensor's sealing washer was fitted if it has been replaced before. A missing or damaged washer lets air past the thread and produces this.
Expect worse economy while it is unrepaired, since this sensor controls the mixture rather than just monitoring it.
What this code means
The upstream oxygen sensor on bank 2 is not beginning its lean-to-rich transition when it should.
This is the last of the eight response codes in this batch, and it is a reasonable place to record what the set is worth as a group. Individually each names one sensor and one direction. Together they describe the shape of a fault — which bank, which direction, upstream or downstream — and that shape is usually more informative than any single code.
For this code specifically, the lean-to-rich direction on an upstream sensor has one cause that stands out: anything letting extra oxygen reach the element. Going rich means oxygen disappearing, so air arriving from somewhere other than the engine masks the transition and delays it.
- An exhaust leak close to the sensor, which draws air in between pulses even though the pipe is nominally under pressure.
- A cracked sensor body, letting its own ambient reference air onto the measuring side.
- A sensor whose seal to the exhaust is imperfect, from a damaged thread or a missing crush washer.
All three are mechanical and all three are cheaper than the sensor. Where a delay is markedly worse lean-to-rich than rich-to-lean, look for air ingress before ordering a part.
Heater performance is worth checking as well. A sensor whose heater is weak sits below its proper operating temperature and responds sluggishly in both directions — so look for a heater code stored alongside, because that would be the real fault.
Likely causes
Start here
- Exhaust leak upstream of the sensor · moderate
- Aging or slow oxygen sensor · moderate
Less common
- Cracked or damaged oxygen sensor body · moderate
- Oxygen sensor not correctly seated · moderate
- Faulty oxygen sensor heater · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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