Check Engine Light

I have an intermitent problem with the "Check Engine" light coming on. I was only happening when the engine was under a heavy load (up long hills, etc). I replaced the plugs and wires, and the PCV and air filter. On the next long trip never saw the light once, but on the return it came on near the end of a 4 hour trip climbing a hill again. It went out almost immediately, but this morning on my short commute it came on and stayed on for a few minutes. I was on a minor hill, but nothing I would have expected to cause this before. I have a 1992 Escort LX, 1.9L engine 286,000 miles.

How old is 02 sensor?

How old is 02 sensor?

I have no memory of replacing it or having it replaced, so I assume original.

I almost forgot. Mileage on the highway had dropped from around 40mpg to near 30. On the first trip after new plugs etc. (w/ no light on) I got 35MPG, but was running the AC most of the time. On the second trip (w/ the light on at the end) I also got around 35MPG w/o AC ever running.

sounds like an oxygen sensor. that will cause an incorrect amount of fuel to be burned in the engine, thus reducing your miles per gallon. go and get the car scanned, even if the light is off now, the computer keeps the error code(s) in memory.