Newbie here - I hope you guys can help me out.

My 2008 4Runner has factory-installed keyless entry and a cheap dealer-installed alarm (the type they put on it while it's sat in their lot - it has always been a bit dodgy). Years ago the central locking was re-programmed by the dealer to open all doors at once. The vehicle has 70K miles on it.

Recently, the central locking has started re-locking itself immediately after opening the vehicle with the keyless transmitter. I press the OPEN button and the vehicle unlocks OK, but then within less than a second it immediately re-locks. To get in the vehicle, I have to press the OPEN button and then immediately pull on the door handle. Even then, the other doors lock.

I have tried two different transmitters - both have the same problem.

Has anyone encountered this problem before? Any suggestions for troubleshooting?

The factory alarm system can do this if it senses that door was not unlocked by a key or key fob (if programmed to do so). Try unlocking door with key and see if they re-lock. Download FSM and read thief deterrent section for more info on setting alarm to passive/ active with forced door lock control. I know you said you have aftermarket alarm but maybe you now have both? or it is controlled by door lock ECU.