Earlier posts here referred to a problem in the wiring harness attached to the knock sensors on the early turbo models. The replacement cost is in the neighborhood of $400 or so, but I'm cheap.

When the "Check Engine" light appears and I get the code for the front or rear (usually) sensor error, I can always get another 7500 miles or so before the next warning by unplugging and then replugging the connections (2) nearest the knock sensors - under the exhaust manifold. I usually record the codes and mileage at the time of occurrence to be sure that I don't have an unusual problem flagged by an out-of-sequence error message. Otherwise, clearing the codes means I'm good for another several weeks, with no ill effects on the performance or operation of the car. Hope this helps.

Eddie H.