Originally posted by ohsigmachi:
I run Techron fuel system cleaner every oil change (6K mi) and the last 2 changes both times I put it in the tank I get about 25 miles through the tank and I get a CEL, 100-150 miles later it goes away. What the hell is this from?

this is my wifes Auto ZX2 (Zetec).



Like high octane fuel, it burns slower and more completely. The codes you are getting are probably lean ones. I've had this problem with mine in the past and it was a prelude to needing to fix something. Dirty MAF, vacuum leaks, bad O2 sensor. The computer probably adapted to the higher octane later.