Fiesta ST MK6 alarm/key problem

[150ST]

I have a 2007 Fiesta ST and the remote key does not seem to work any more, meaning after unlocking the car using the key rather than remote, the only way to turn the alarm off is by turning the ignition on. I have tried replacing the remote battery which didn't solve it. Any suggestions on what might be the problem? New remote key needed or something wrong with the car's locking system? Has anyone had a similar issue?
I would probably just put up with it but I'll be selling it in the next month so could do with getting it sorted.

[LuS1fer]

I had two keys with mine and one didn't have a remote, just the key. This always set the alarm off until the ignition was switched on.
I would like to provide a logical conclusion to that point but can't.

[S0 What]

try re syncing the key



Get in the car and close all the door but don't lock them

Switch the ignition on/off 4 times within 6 seconds then leave in the off poss, you should hear a bong or beep to confirm you are in programming mode.

Press any button on the key fob within 10 seconds of hearing the bong/beep and you will hear another bong/beep to confirm its programmed successfully.

Switch ignition on for 10 seconds to exit programming mode