I have an O/R X-pipe installed on my car without MIL eliminators. I had a diablo tune on the car to disable the o2's but the car ran crappy with the tune. I took the tune off and had been running 89 octane and my CEL has been on for months. However, I decided to fill the car up with 91 octane and the CEL turned off within 5 miles and has been off ever since. Does anyone know how 91 octane fixes this issue?

 

What octane fuel was the tune created for?

OK, so the SES (service engine soon) light was on after you removed the tune and returned the stock tune, then it shut off with the higher octane fuel while still on the stock tune? That's weird. Was the car knocking with the 89 gas?

 

Yea I removed the tune and the CEL came on. I put in 91 octane and it went away. I didn't notice knocking.

 

Its just a fluke, rear o2 sensors do nothing as far as engine managment is concerned. All they do is act as a check to ensure the CATS are working properly, they in no way can effect gas mileage or how the car runs. To start what was the CEL code, it might not even have been an 02 sensor related code.

 

Could have been the gas cap was loose or improperly seated...

 

so what was the actual code stored in the ecm? without that it could be anything. We're just guessing.

 

Well the check engine light went on the day I had my O/R x-pipe installed about 2 years ago and has been on ever since. I have always ran 89 octane. I decided to put in 91 octane and the SES light went turned off about 2 weeks ago. I finished that tank and filled up the tank again with 91 octane and it has stayed off ever since. I always just assumed the SES light was on due to the O/R X-pipe.

 

Did you ever check the code(s), then or of late?

 

Maybe the bulb just burned out?

 

+1, just get the codes ran then you know for sure whats going on.