yes, you can have codes without having a Check Engine Light on. Its usually the more serious codes that will turn the light on. Since replacing my engine I've gotten codes for the KS several times, "P0328" (Knock Sensor 1 Circut High Input (Bank 1)) that did not turn on the CEL, but did not have any issues with how it ran with that code.

I have had the CEL turn on for "P0430" (Catalyst System Efficiency Below Threshold (Bank 2) ). again , guessing this was a more important issue since it turned the CEL on, but did not affect how it ran at all. I've done a SeaFoam treatment and havent had the last code come back since.

If you do get the codes read, make sure you write down what they are and search them on the web to get an idea whats going on, or bring it back here and ask, lots of info for it out there.