I had an emission light activate on the motorway one day I knew nothing was wrong but I have a Renault clip and there is a test for the catalytic converter that runs the engine at 2500 rpm for about 2 minutes and must alter the ignition and fuel during that time to heat the CAT like a regen and calibrate the lambdas. Now since the car had a recent battery failure it looked like the injection computer had frozen and took me a while to get the car ignition right to clear the fault as it would not clear with ignition on or the car running so I stalled it to keep the red ignition lights on and now I know you insert the keycard and hold the start button for 5sec or so to get it there. Anyway without touching anything I cleared the ignition fault code ran the catalytic converter test the lambdas started to show live varying readings and the emissions warning disappeared. The wet plugs if they were not firing you would feel it the other thing would be if the injectors were stuck or leaking wetting the plugs and will destroy the lambdas. If you have just fitted the decat and the manifold is not genuine then there is no CAT's fitted I would expect the light to come on.