Security system went funky

[rockhead]

Have an OEM security system in my '05 2.3 sedan. Couple of days ago the security lights-off when I use the fob to open the drivers door. Get that under control by opening the trunk lid with the key, but then it goes off again when I tried the remote starter (also OEM). This time it also tripped the immobilizer (red flashing light in dash).
Car is operational and I've diconnected the horn +ve wire so I don't get a neighbourhood posse after me, but it still trips the immobilzer when using the remote start function.

Any ideas? Could it be one of the door sensors? It's been working fine up until now.

[Nevermind]

Have an OEM security system in my '05 2.3 sedan. Couple of days ago the security lights-off when I use the fob to open the drivers door. Get that under control by opening the trunk lid with the key, but then it goes off again when I tried the remote starter (also OEM). This time it also tripped the immobilizer (red flashing light in dash).
Car is operational and I've diconnected the horn +ve wire so I don't get a neighbourhood posse after me, but it still trips the immobilzer when using the remote start function.

Any ideas? Could it be one of the door sensors? It's been working fine up until now.

Remote starter you say is OEM, installed at the factory or the dealer? If the dealer than you have an after market remote starter which could be your problem, disconnect it and see what happens. I had some sort of the same problem but I don't have a remote starter. The horn would sound after closing the trunk, to stop the horn I would lock and unlock the doors but not using the FOB. Not sure if this helps.
Good luck

[S.F.W.]

There is/was a Mazda branded remote starter . That said, it was not a factory option, but may have been dealer installed.

[rockhead]

Yeah, the starter was added on at the dealer, so I'm guessing it was an aftermarket as you had mentioned.

[TheMAN]

the mazda remote starter kits are all dealer or port installed... it was made by code alarm and their reliability hasn't been great
you will probably need the dealer to diagnose this system as it is fairly complex