Ford Repair: Check Engine Ford Probe, mechanic shop, mazda engine

Question
Hello Sir,
I Own an automatic Ford Probe SE. Its a 1993 model. It has 4 cylender Mazda Engine. Couple of days back Its "Check Engine" sign lit-up. So I stopped on side and switch off the car. When I restarted it the check engine light was off. But now whenever I drive 10-12 miles at speed 30-45 miles/hr or 5-6 miles above 55 miles/hr check engine light lits-up again. Also the engine oil level when I start is normal but as i drive it keeps falling towards low. When I switch off the car it comes back to normal and keeps falling when I dirve again.
The car was running fine. But after a week or so its engine started vibrating a little at speed over 55mile/hr.

So I took it to a mechanic shop. They said computer is not showing any kind of error code. Also they checked few things such as O2 sensor, radiator etc.. everything seems to be fine. So they returned my car saying there is nothing wrong with the car.

I am not sure what should I do? shall I take it to another mechanic or is there anything I can do by myself.Any help on this will really be appreciated.

Thanks

Answer
By disconnecting the battery you clear the codes on pre-1996 vehicles.There are only one or two sensors that go off and on like that.None of them are very serious.The o2 sensor is the likely culprit.Now as far as the falling oil pressure,this is pretty normal.Short of rebuilding the engine and replacing the oil pump,don't worry.Just keep doing what you are doing,keep you're tune ups and oil changes on time.
Oh,by the way,very eloquent question,I wish they were all that easy to read.

The car will be fine until you get a chance to replace the o2 sensor.However, it will affect fuel economy.