Hello there; Thanks in advanced for your assistance.

I replaced the heater core, following you-tube videos. Put dashboard back together, and now the theft light flashes, and turns over but wont start. All the electrical connectors under dash are plugged it. Did my key die, or what did I do to the theft system? Truck is in son's garage in Old Fort TN, by Cleveland. Any help is appreciated, as I need truck back for work.

 

is the theft light flashing rapidly?

 

My son said it flashes rapidly, then it slowly flashes a code, but it very faint and difficult to read. He said he thinks it flashes code 16

 

Here's what it is supposed to do.

Key off and key on.

https://drive.google.com/file/d/1Y17...usp=share_link

 

What year is the truck? PATS keys don't have a battery. Have you tried both sets of keys?
1) make sure you are using the correct key to start the vehicle, it must be a PATS key programmed for your truck.
2) when trying to start the vehicle make sure that no other metal is touching the ignition switch while trying to start the vehicle. Sometimes another key that is attached to the key chain will cause a problem starting if it also is touching the ignition switch.

The THEFT light will flash every 2 seconds when armed, when the ignition switch is turned on the THEFT light will remain on for 3 seconds and then go out.

 

Truck is 1999 F150 XL, 2WD with 4.6 Triton.
Only have one key for truck, no other keys on key ring.

 

When you turn the ignition switch to the on position does the THEFT light go out after 3 seconds?
If it stays on, the THEFT system is not recognizing your key. Let's hope it something simple to fix.

 

Most likely is a wire pulled loose when the dash was removed. There's not much to the system. The transceiver ring has power and recognizes the key, it sends the key code to the cluster and PCM, the PCM allows starting to happen. If you didn't break the cluster or the transceiver then there's a wire disconnected or a blown fuse. I'd check all of the areas that were touched during the repair.

Looks like Fuse 30 but it controls quite a bit more than just PATS.

30 30A Passive Anti Theft Transceiver, Cluster, Ignition Coils, Powertrain Control Module Relay

 

p.s. what the light is doing with the key out is probably a clue. It would help your cause if you described the sequence of events from key out to in to On to Start.