Won't go into 4H or 4L. Push button, 2H lite blinks as normal then goes out. Can hear the usual "click" under dash, but no 4X. Have checked all fuses. Tried grounding pin 13 for diagnostic code (on 4X button lites), nothing. Has anyone else successfully diagnosed one of these systems before? Is there a step by step somewhere on the net? Any advice before I take it to the shop? Thanks.