Theft system help

Have a 99 Ford Ranger with the 4.0 and manual trans. Last week one eve when I went to start it it just cranked and theft light flashed rapidly. After switching it on and off several times it went out and started right up. No issues rest of that day. After sitting over night the next morning same issue. I took it to a ford dealer and they told me to replace the antenna ring around key switch. I did that and still same issue. Only does it early in the morning. After it heats up outside it starts everytime with no issues. Seems as if it’s a moisture problem of some sort. Has anyone ever had this issue and can give me some advice on where I need to look next? Dealer saying I need a new computer but don’t want to go that route unless completely nessacerey. Thanks in advance. Also I’m new here this may have been covered already and if so I’m sorry I couldn’t find it anywhere in the forum unless I over looked it.

Welcome to TRS

The system is called PATS, passive anti-theft system

In a 1999 Ranger it will have 3 parts, 4 counting the PATS key
Transceiver, the part you replaced
PATS Module, located above the glove box
PCM, power train control module, "the computer"

Embedded in the Keys handle is an RFID tag, radio frequency identification tag
When you turn the key on the transceiver sends out an RF burst, this gives the RFID tag just enough power to transmit its unique number, and the transceiver picks up that number and sends it to PATS module.

Just to be clear here, the transceiver is just an antenna, no "electronics", the PATS module sends out the RF Burst, and Reads the number via the transceiver/antenna.

The PATS module has a memory of the RFID tags it was program with at the factory, so 3 or 4 RFID tag numbers, whatever number of keys the truck came with when new.
These tag numbers can also be stored in the PCM, depending on PATS version

It compares the the RFID tag number it just got from the transceiver to the numbers in memory, if it finds a match then it sends a unique code to PCM, code changes every time you try to start engine, just FYI, PCM then turns on fuel pump and fuel injectors for starting.
If no match is found the Theft Light will flash rapidly and no fuel pump or fuel injectors, so no start
Later model PATS also added starter motor disable, so starter motor wouldn't work either with a "no Match"

RFID tags don't need batteries so last a long time but they can start to garble the number they transmit.
If you just have the one RFID tag key then I would contact local locksmith to see how much to add another key to the system
Locksmith needs to make/cut the key and then use a laptop with Ford software to add the new key's number to PATS/PCMs memory.

Locksmith can also "clone" your current PATS key
He uses a Blank RFID PATS key, he reads the number of your current key and then duplicates that number on the Blank RFID tag
This is cheaper but the downside is IF the PATS module or PCM has corrupted number for that key then new key with same number will suffer the same fate, lol.


If you have a second key then try it.


PATS module and PCM are "married" you can't swap either out to test it
If either needs to be replaced then ONLY a Ford Shop can "remarry" these.
The "marrying" is to set up the RSA encryption they both use.

You could swap both from another vehicle but then you also need that vehicles working PATS keys because they are in that sets memory not your key(s)

One "heads up" if you have two or more Ford vehicles with PATS and have the keys on the same key ring then that can cause "garbled" read of the key in the ignition, so best to use separate key rings, PATS key needs to be within a few inches of transceiver to be read correctly, the RF Burst goes farther, so can activate other PATS keys to send out their number