Not sure, I've not taken it apart to look at it because it's got a two year warranty... though finding out where to contact them is proving to be troublesome.
They advertize sealed and appear sealed... this looks interesting:
Oxford Heated Grips issue - ADVrider
I'll have to make sure all my connections are tight, I did recently do my clutch/throttle cables, possible I slightly pulled one of them out of the socket... it just seemed strange the power light would still come on, but no indicator of heat level.