I hate the fact that these transponder keys are so big and having several cars in the family makes the key ring really big.
I decided to try an old trick my father and I used on our older trucks... Cut one side of the key to one vehicle and the other side of the key for truck Two. After fighting with the key person at home depot for 10 minutes (her name tag said "Jingles") she finally cut the key telling over and over and over that I was wrong and it would not work. Hmmmm...I think see must be related to my wife in some way.
She was half right. IT WORKS on the F150 but does not work on the Five Hundred.
Plan B....I got a regular small key cut for the truck. Went into the programing sequence and when it was time to program the new key... I used the regular key but had my wife's transponder key next to it on the ring.
Just thought I could help same some real estate on your key ring using either method.

 

LOL jingles. Sounds like someone that could use a swift kick, if you catch my drift. Interesting to know it works in one and not the other... what years are they? I drive an 01 F150 and the old lady has an 06 Fusion....

 

But don't both of these vehicles have chips in the keys ... specific to each respective vehicle? That would mean you couldn't have one key work both vehicles. And how often do you really find yourself needing both keys at the same time when away from home? Just have one keychain for one, and one for the other ... like my folks have done forever. And finally, leave the key hidden in the vehicle and use a keyless entry keypad for entry. I did this with my Maxima for five years ... never carried my keys on me at any time.

 

Jingles is right. Your key has the MicroChip in it and will only work for the vehicle iit is programed for

 

Seems to me you just need to program the key to both trucks. From what I understand when you program a key it just reads the radio id of the key and records it. I don’t think there is anyway for it to know it’s been programmed to a 2nd truck as well.

Found this: "PATS contains a feature called unlimited key mode. This feature allows a customer to program more than 8 vehicle keys, if requested. Each vehicle in unlimited key mode is set up with a special unlimited transponder security key code. This allows all the customer vehicles to share the same mechanically cut keys. For an individual customer, any randomly selected security key is acceptable."

Not sure I fully understand it.

 

I have three Ford products with the transponder keys and remotes and have the same problem. I resolved the problem by using a drawer that is next to the door going into the garage. I have each key and remote on a separate key ring with a house key. I leave all the keys in the drawer except the one for the vehicle I am using (can't drive but one at a time anyway). If I switch vehicles, it is only a 10 foot walk to switch keys.

 

This seems like a possible solution. One Transponder key on your key chain that is programed for each vehical you own. Then cut regular keys for the other vehicles and leave those keys in the vehicle or on your keychain. If each of the vehicles recognizes that one transponder key on the key chain it shouldn't matter that it's not the one in the ignition. If you leave a regular key in the vehical, just clip the transponder on the key chain so it is close enough to the ignition to work.

My wife drives an 05 Freestar and I am getting an 04 F150 Lariat Supercab on the 24th (Feb 2007). If no one else has tried this by then I will give it a shot. Otherwise, if you try it please let us know how it goes.

J

 

I do not know if some of the previous posters understand the PATS working theory.
The key is not programed to the vehicle. The Vehicle is programed to recognize the key. You can have one transponder key and it can work unlimited vehicles with the PATS system.
I was having Jingles cut a two sided cheap brass key to see if the Tumblers in the lock itself would work. If it did I was going to have a transponder key cut the same way. If I had two F150's the one key would have worked on both trucks just by flipping the key over. Jingles' argument was about the 7 cut key needing both sides to operate the lock cylinder.
The way I am doing it now is just having my F150 read the transponder of my wifes' key that is next to my $1.99 brass key (also much smaller)on the ring.
Scott... The vehicles are 07 F150 and 07 Five Hundred.
I do not know if I would like to live where every vehicle I ever wanted to use or move was ten feet from my front door. Guess that would not work on a farm. Then again on most farms you can just leave the keys in the truck.
Johnshiredman....Your solution is what I am doing right now.