Possible O2 Sensor Issue?

My 88 Lx-i will occasionally throw the check engine light while driving on the highway. The problem is that it goes out before I have a chance to stop and check the lights on the ecu. I've only been getting 23-25mpg mixed driving which seems low (even being an auto) compared to what I've been seeing other people on here get. The car runs smooth and has decent power so I don't suspect a vacuum leak. I do notice from time to time it slightly hesistates when leaving from a dead stop but nothing serious. My initial guess it that I've got an O2 sensor on the brink. Car has 186k miles and I have detailed service records from the original owners. I don't see where they have ever been changed. The only other thing I can think of is that the distributor was replaced not long before I purchased the car. I'm going to check the timing to make sure whoever worked on it set it back correctly. What do you guys think?

Get someone to sit in your back seat and when the light comes on have them count the led blinks on the ecu.

Ok so I checked the timing tonight and it was about about 10 degrees retarded. The hesitation is gone and the car feels peppier, hopefully my mpg will follow.

With ignition key on, look under the driver's seat from the rear. There is a small cover for the ECU led. Pull it gently, and count the flashes. Since yours is an 88 it has two oxygen sensors so there may be two different codes.

If you have no record of replacement, replace them both. They are inexpensive (about $20 a piece from rockauto.com). Or you may buy "universal" ones and splice the connectors from the old ones to the new ones.

Ignition timing should be 15 degrees BTDC - the red mark in the flex plate.

I was checking those out earlier and I think I'll probably go ahead and swap them. I've got the timing set to 15 exactly. It was pretty far off when I put the light on it. Vacuum advance responded well to taking the hose on and off to set the timing too.

Code ended up being the speed sensor. Thank goodness it was something cheap ;)