I have a 2001 F150 bi-fuel CNG. Truck has 80K miles, and I just flushed the CNG with isopropyl and put a new coalescing filter on. Truck runs well on regular gas, and well on CNG if alt light is on. My problem is that the CNG light only comes on occassionaly. It never flickers, or flashes. It is either on or it is off. Sometimes I can start the truck on CNG for a minute, shut it off, and when I restart it, the CNG light is off. I recently stopped for lunch, truck was on CNG. Before I got out, I shut engine off, and restarted on CNG as a test, it worked. When I was done with lunch 20 minutes later, CNG would not activate, even though truck hadn't moved an inch. I've checked all the fuses, including the 2 on the battery block, and they are all good. The CNG system seems to turn on more often when it is warm out, but even then it is sporadic. What might explain this?
Try to turn the key to the "on" position (battery powers the radio and dials but engine has not yet received ignition) and wait for the green light to show up. If it doesn't show up, turn the truck off and try again until the light turns on. Once the green light is on, turn the ignition. Check out my other thread on here.