I just want to have some clarification.

Although my CEL (check engine light) have not turned on yet, I was thinking of having my ECU scanned because of some weird behaviour of my engine while running.

My questions are:

1. can the ECU scanner (MUT) find/read/detect an error/problem if the CEL is not on?

2. can it only detect errors if the CEL is on? If so, I guess there's no point having the ECU scanned if the CEL is not on, tama ba?

Thanks very much.