Recently my CEL came on. I took my wrx to the dealership and they replaced my radiator, (which I thought was causing it). Then they told me that my CEL would not cut off because of my exhaust system. I got a b&b high flow downpipe and blitz nur spec. Could that be the reason for the CEL??
Strange that they changed the radiator. I might look into a different place to have my car serviced. Seems like if you are getting a code, the first thing they would do is pull the code and see what it is.
But, yeah, I'd be willing to bet it's your downpipe O2 sensor causing the CEL (the P0420 code Joey mentioned). Easy enough to fix.