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Having a problem with my passenger door central locking. When I push the remote to lock the car only the drivers door locks, if I push the remote a couple of times the passenger door finally locks. This also plays up when unlocking as well, any suggestions out there? My car is an 04 SE.

Factory system?

Take the door skin off and have a look around. Try locking/unlocking it, both manually and remotely, a few times when you can see the actuator and it should be fairly obvious where the problem lies if it's in the door.
Wiring issues can be less fun to trace but a few minutes with a multimeter back probing the connections and you should be able to isolate the problem.
Drivers side works so it's not a problem with the remote itself.

Yeah, it's the factory system, will take the skin off in the morning and have a look around. Crapweasel on the Mazdaspeed forum says it might be a low battery in the remote, didn't know it sent the signal to both sides, if it does it might not be strong enough to send the signal to the other side of the car.

Nope. There is only one receiver in the car. If the drivers side is working then the remote is working.

There could however be a wiring issue between the receiver and the passenger door, or a problem with the receiver itself in that it only sends the signal to one door, but the remote itself is fine.

Sorry I dont have a complete solution but these might help

http://mx5cartalk.com/forum/viewtopic.php?f=55&t=34235&p=463298

take the trim off and listen to the actuator when locking/unlocking.. if you can hear the mechanical noise but not lock/unlock action, then most probably problem is with your actuator.. if you can not hear anything, get yourself a multimeter, and see if you are getting any voltage when locking/unlocking... then trace it up towards the receiver to isolate the problem..

Where would I find the receiver?