1998 Ford Contour check engine light

Dont be mad at your mechanic. 1st off, when you have a check engine light, the tech scans it for a code. 1 code can have 10 different fixes. It happens. Testing certain parts can reduce the # to about 2-5. At that point its educated guessing. Your check engine light goes off for 2 weeks because he clears the codes, as he should. What is the code? Bring it to your local parts dealer and have it scanned. I see this alot with a p0420 code which is cat efficiency. Normally you replace the 02 sensors 1st, seeing as they tell your computer if the cat is good. If the code comes back, then you need a cat (usually around $500) so we always try to go with the cheaper alternative for customers. Thats just an example. We cant just say whats wrong with your vehicle every time and be right. Even the best techs are wrong sometimes.
What is the code number being stored?

That means the O/2 sensor is reporting too lean of a fuel mixture. That can mean the sensor is faulty but it usually doesn't. Fords are pretty notorious for bad vacuum leaks causing that code. With the engine idling and the hood open, listen very carefully for a hissing noise and try to follow it to it's source. They are notorious for sucking holes through rubber connectors to large vacuum supplies like the PCV or brake booster.

This is a serious issue but should not be that hard to solve. If you current shop is having that much of a problem, then you just need a different shop. These are usually solved in the first 15 minutes of looking at a car.