I am not sure about the 420 series but I bet it is in the same location as my 380. Check on the firewall about half-way of the engine. It is a black box about 4 by 11/2 inches and is held in place by a 10 mm screw. Loosen the nut( careful not to drop it) and pull the module and its harness out. It pulls away from the harness. The solder joints in these things are brittle and constantly fail. You can try to re-solder them or just buy a new one. I paid $250. It was worth it.