P015C: O2 Sensor Delayed Response - Rich to Lean Bank 2 Sensor 1
What to do
Ask whether the same fault is stored on the other bank. Both banks together means the fuelling itself is at fault rather than the sensors, and replacing two sensors will not fix it.
If any oxygen sensor work has been done recently, ask for the connectors to be confirmed as going to the right banks. The two sides often use identical plugs, and crossing them makes each bank trim from the other's sensor — which produces strange readings and is corrected in minutes once spotted.
Ask for an exhaust leak check on this bank, which is a common cause and often audible when cold.
Expect worse fuel 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 rich-to-lean transition when it should.
This is an upstream sensor on the second bank, so it controls fuelling for that bank alone — and on most V engines the two banks are trimmed independently. That independence is what makes a comparison between them so useful.
Bank 2 running late while bank 1 responds correctly is a bank-specific fault, and the list is short: this sensor, an exhaust leak on this side, or the wiring to it.
Both banks late together is not a sensor problem at all. It means the mixture really is arriving late relative to what the module commanded — a fuel pressure problem, a purge valve, or a fuelling strategy fault — and both sensors are reporting honestly.
One trap belongs specifically to upstream sensors on a V engine, and it is worth naming because the symptom is bizarre. The two banks' sensor connectors are often identical and within reach of each other, and when they are crossed, each bank is trimmed from the other bank's sensor. The result is a pair of banks chasing each other's mixture, with fuel trims running to opposite extremes and codes that make no sense individually.
After any work involving both sensors, confirm each connector goes to its own bank before diagnosing anything else.
Likely causes
Start here
- Aging or slow oxygen sensor · moderate
- Exhaust leak upstream of the sensor · moderate
Less common
- Oxygen sensor connectors crossed during repair · easy
- Contaminated oxygen sensor element · moderate
- Metal debris on the sensor tip · moderate
Symptoms
Components involved
Information, not instruction
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