My problem was the lights would just flicker when turned on. Tried replacing the florescent tubes and still just flickered. I bought this and replaced a 20 year-old Advance Mark III R-2S40-1-P ballast for two florescent bulbs. I cut the eight wires a couple of inches from the old ballast and took it out. I had made sure the length of this ballast was the same (9.5") and it fit just fine where the old one was with no modification. It had the same eight colored wires (2blue, 2red, 2yellow, 1black, 1white) as the old one did so I just matched them up, stripped 1/2" off each wire and connected them using medium wire nuts. It works! Turns on instantly where the old ballast had a lag. We'll see how long it lasts.