This 2001 Chevrolet Silverado came in the shop with the complaint of no stoplights. First I confirmed that none of the stoplights were working including the CHMSL (center high mounted stop light). After this I went to the underhood fuse box because the manufacturer has made diagnosing the stoplights very easy at this location. By going to the red, 10 amp, vehicle stoplight fuse, #17 and checking for power while some pushes on the brake pedal I can tell easily if the stoplight switch is good and I do not have to lay under the dash to do it. I identified the fuse location using the legend.
This one was really easy because when I located where the fuse was supposed to be I found that it was missing. I installed a new fuse and tested both sides for power while someone depressed the brake pedal. I found that the owner had been swapping fuses trying to keep his power outlet working for his cell phone charger.
If I had found power on one side of the fuse but not the other it would have indicated a blown fuse. If I had found no power on either side with the brake pedal depressed I would have suspected a faulty stoplight switch or no power to it and more testing would have been needed.
Luckily, the power supply fuse for the stoplight switch is the yellow, 20 amp fuse, #14 is right above the fuse I was testing. Testing here would have shown one of two things. Power on fuse #14 with no power on fuse #17 would indicate a faulty stoplight switch. If fuse #14 had been blown it would have indicated a short between the fuse and the stoplight switch and back to fuse #17. Also a blown fuse #17 would indicate a short between the fuse and the rear of the truck.