I have a 2004 f150 heritage, which I believe is the same as the 2003 heritage (it is different than my friends 2004 f150).
My truck was/is dual fuel (gas/propane). The propane switch and tank had been removed when I got it, however the under hood modules and wiring are still there.
Several years ago I changed the spark plugs, and the computer expects all that to be connected to start.
Recently the 5.4l blew the number 3 plug. The truck has over 220k miles, but has been good and reliable, and I am too stubborn to scrap it.
It took me 2 weeks to complete the bore and install the inserts and new plugs. All connected - key on to check fuel leaks - flashing THEFT light.
I didn't know it had an anti-theft system.
I have searched different threads, and tried a few things, but no change.
Key on 10 minutes - nothing.

 

A rapidly flashing theft light indicates the Passive Anti-Theft System (PATS) is not recognizing the key, leading to a no-start condition. Common causes include a damaged key chip, a weak truck battery, or a failed transceiver.

 

Observe the odometer when the ignition switch is turned from OFF to RUN. Report observation.

 

Thanks for replies.
Further info - the truck was running fine. We had stopped at a restaurant. When I started it the number 3 plug blew out. I drove it home.
The next day I drove it into my shop and disconnected the battery. Ordered tools, inserts and new spark plugs.
Note: this is a dual fuel engine. It is a lengthy process just to access the spark plugs. I took about two weeks to complete the job.
All connections checked, battery connected, key on. Safety note: anytime the fuel rail is disconnected, always turn the key to on for a few seconds - not start - and check to make sure there are no fuel leaks or possible fire risk. The flashing THEFT indicator and dashes in the odometer prevent the fuel pump relay from activating.
I do not see how sitting for a few weeks would affect ground connections or relays. The key remained on a shelf with two other car key fobs.
Can other key fobs affect the chip in the Ford key?
Any thoughts on what to try next?

 

See and respond directly to post #3.

 

The flashing THEFT indicator and dashes in the odometer
Note in response #4.

 

It was not clear, came across as a mish-mash.

In any event, you do not have a PATS system failure, per se. That is a secondary symptom.

Your PCM and the instrument cluster did not power up together and establish communications within a very short timing window. The most common reason is a PCM Power Relay that has failed and the PCM didn't power up. Swap that relay with an identical relay that is not critical to engine operation and test again. If it still fails, it will be necessary to dig in deeper.

 

projectSHO89 thanks for your reply.
The PCM relay and the fuel pump relay are the same.
I also had an unused relay. Results were the same with all 3.
I even left both relays out and the result is the same.
I feel at this point that it has something to do with the propane system.
I would like to remove all the propane equipment, but the computer expects it to be there.
I think that I will try to find a wiring diagram for the dual-fuel system.
I haven't given up (yet).