Buy a “test spark plug” and a can of starting fluid. When it’s in the “no start” mode, connect the test plug to one of the spark plug wires and crank it. Do you have ignition spark?? If no, then you have pretty much found your problem. If yes, spray some starting fluid into the air-cleaner box and crank it. It should start and run momentarily…This tells you the engine is not being supplied with fuel…
You mentioned an anti-theft system…That’s a likely suspect assuming it’s more than just an alarm…
Visit this web-site, www.crownvicnet.org There is a forum specifically devoted to the 4.6L engine and these guys are pretty sharp…