Why doesnt my 2000 ford f150 start up when switched to propane

presuming you have fuel in the tank, you need to check fuel pressure. Or first physically check you do not have pinched fuel line and fuel filter is new for propane as well as propane pump is working correctly.

sounds like your ignition switch under steering colum has gone bad autozone or dealer should have new one if this was helpful please leave me good feedback

start with the fuses and relays if it has a lock out switch that would be the next test

If the anti-theft light is flashing, I would think it is a problem with the key or the anti-theft system. The system would kill the power to the fuel system.

AW MAN, YOU GONNA HAVE TO TAKE IT TO GET REPROGEMAMED TO THAT CLUSTER, OR PUT THE ORIGINAL BACK ON AND GOOD LUCK STARING IT. IF NOT TOW IT IN AND HAVE IT REPROGRAMED

There amy be a problem with the heater relay switch which is not allowing it to switch over to heat.

sounds like a vacuum leak i have run into that several times. however i haven't found any common place for the leak. i have found some under the dash, on the back of the intake manifold, and i have found them at vacuum resivors. i wish you luck. usually you can find leaks easily using propane. rarely i have seen a bad check valve cause this problem, but that is a possibility as well. hope i was helpful to you.. dustang

behind the passenger side kick panel is an inertia switch,press down on the red button which will reset it,if it doesnt start listen to hear the fuel pump

Check the fuse for the cigarette lighter, most car makers use the same circuit for power to the OBDII connector.

if it is a 134a system the port should be on the right side near the drierit should have a blue cap on it the high side should be near the front of vehicle near the condensor. the low side pressur should be 30-35 psi high side should be 130-150psi. do you have quick connects?---are the gauges for 134a it should say on the face of the guage.