I have a '96 Impreza Outback Sport with the 2.2L motor and 123k miles. Recently, the check engine light will come on and stay on for a few days, and then go out for a few days, and then come back on for a few days, etc., etc.....
It hasn't bothered me too much because when the light is on, the engine seems to run fine - starts OK, idles OK, accelerates OK, not rough, etc., except for fuel consumption. The mileage goes way down. I can actually notice a drop on the fuel guage in my 7-mile commute. When the CEL is off, mileage returns to normal.
I'm going to take a shot at the possible problem, and I'd like to get others' opinions.
I'm guessing it's either: 1. A bad oxygen sensor (they have never been replaced) or, 2. MAF has gone bad. My reasoning for this comes from what I know of the engine computer and how it is programmed with a "limp home" mode when something goes wrong. Considering that my two possible culprits are both "input" devices to calculate the fuel/air ratio by the engine computer, when the CEL is lit, is the computer in a "limp" mode, possibly using a default, overly-rich fuel/air mixture as to keep things from getting too hot, thus the high fuel consumption?
It's just a guess.
Thanks, Christian