Yes, tell them you have a CEL and they'll scan it. Clearing the ECU basically means pulling power to the computer so that any stored error codes are erased. The computer stores these codes even after you've corrected the problem and the CEL will remain on. For some error codes, the computer runs the car in a "limp" mode which can be detremental to performance and shorten the life of components. It is probable that your car is in limp mode now.
The only two ways to clear the code are through disconnecting power to the computer or by using a code scanner. Keep in mind that clearing the ECU without correcting the problem will only cause the CEL to come on again when the system detects there is still something wrong.
