Just fitted a full Janspeed Decat to my 03 WRX.
On the way home last night an orange warning light appeared on the dash.

Me thinks this is telling me the emissions are too high - which you'd expect
after fitting this exhaust.

Should I be worried - And how can I switch it off???

 

If its a full decat it could need a CEL fix.
When a cat downpipe cat is removed it removes a sensor, you need to trick this.
Do a search on 'CEL fix'

 

I take it you was doing a constant speed between 60 and 90mph for the first time since fitting the exhaust? The sensor requires a constant speed so it can monitor the cat, but because you have removed it (mainly up pipe cat), it doesn't detect one and so the CEL light appears. You shouldn't worry about it, but as mentioned a CEL fix can be purchased to remove the light.

 

Yeah - the light came on after about 20mins on the motorway.
Glads it's nowt to worry about. I've spent enough cash on this thing already!!

 

There's some instructions on here somewhere of a 26p fix for the CEL, or you could get a 2nd hand unit for around £10 if you wanted to get rid of the light.

 

I would still get the error code checked just in case