2003 Ford Focus - Check Engine Light on but mechanics can't find anything wrong

My car has had the “Check Engine Light” come on 3 times. Each time it clicks back off after 4-5 drives. When I drive it the headlights will dim in and out on their own and after 30-40 minutes of driving it will “stutter” or “shudder” underneath me. It get’s to the point where I have to pull off and stop driving otherwise I’m afraid it will break down. I’ve taken it to the mechanic and they said that there is nothing wrong will the wires or alternator. The check engine light just came on again and I’m not sure what to do. Help!

Take the car to an auto store and have the diagnostic codes read (the check engine light indicates that there is at least one code stored). Post the codes here and you will get some good advice.

Besides getting the codes read, if this were my car I’d hook a DVM up to the +12 v battery voltage (usually this is available on a wire or connector in the passenger compartment somewhere, I’d refer to the wiring schematic) and just monitor what it reads as I drive the car. It should be 12.5-13.5 volts when the engine and accessories are turned off, key in “off”, and 14-14.5 volts when the key is in “on” and the engine is running.

Battery cable terminals that were badly in need of cleaning used to cause similar problems. Maybe you’ll be lucky.