Somewhat replicable issue.
Scenario:
I drive to a destination, turn the vehicle off, DO NOT remove the key, turn the key back to run position, theft light code 16.

this is only a scenario I replicate, sometimes it happens randomly and I can not turn it off by simply locking and unlocking the doors, sometimes I have to let the truck sit for an extended period of time before it starts back up.

is this normal or an issue?

cleaned engine bay grounds, could it be an old pcm relay? Bad battery cables?

 

Welcome to the forum

First, PATS is a passive anti-theft system, so not an alarm, it has no connection to door locks or Fobs

1999 V6 Ranger will have a separate PATS module located above the glove box, this matters for code 16
With key on PATS reads the RFID tag in the key's handle(via transceiver), if that tag's number matches a number in memory the PATS module sends the "OK to start" message to the Computer(PCM) in engine bay via the CAN network, 2 wire digital network(1's and 0's)
(This message is encrypted with a Shared Key, RSA encryption, so can't be duplicated by thieves, also means you can't just swap out PATS modules)

PATS codes
11 Transceiver [transponder coil] not connected, or there is no power to it.
12 Transceiver internal fault.
13 No Key/no PATS key has been used to turn the ignition switch.
14 Incomplete key code received.
15 Invalid key code received.
16 CAN communications error.

16 indicates PATS module and PCM could not communicate at that time over the CAN network

Shouldn't be a key issue or voltage issue

If you have or can get an OBD2 reader see if you can communicate with the PCM, that is done via the CAN network from the OBD2 port under the steering column area
If it is an intermittent connection then pull off the PCM's 104 wire connector in engine bay and look for water intrusion, clean it out as best you can and reinstall it, just the removal of the connector and re-installing it cleans all the pins and slots regardless
The tan/orange and pink/blue wires at pins 15 and 16 are the CAN data wires

If no improvement with PATS connection then you will need to access the PATS module by removing the passenger side airbag

Picture of its location: https://www.ranger-forums.com/attach...n-img_5233.jpg

Unhook the Negative battery cable and wait 15min before working around ANY airbag, they store an electrical charge and can deploy accidentally

You will unplug and check the connector, same as PCM's connector
Same tan/orange and pink/blue wires are at pins 5 and 6 on PATS module

And same tan/orange and pink/blue wires are on pins 2 and 10 at the OBD2 plug-in, its the CAN network