My Dad has an 07 Fiesta, two door. Randomly, the 'passenger door open' warning pops up on the dash.

Door is secure and I can see no switches on the door or frame.

I'm assuming a faulty latch, but could the fault be elsewhere ?

Almost certainly the microswitch in the door lock mechanism.

We have a 2010 Fusion and a 2007 Fiesta, I have changed two front passenger door locks, one in the Fiesta, and one in the Fusion.  Also changed an offside passenger door lock in the Fusion, and I have the nearside to do !

It may be worth disconnecting the connector in the "A" post first and checking that the contacts are clean, before you embark on the hours of endless fun accessing the (very expensive new!) door lock

Thanks for the input. I will start with that A post connector and cross my fingers.

Let the forum know how you get on and what your plan will be.

Had the same issues on my sons Fiesta, believe it or not it’s easier to change the whole door than the door lock.

We did it on the drivers side as we had a barrel lock to replace anyway, about 98.99% of the door needs to come apart to access it, there is a few guides on YouTube to help. 
 

I checked the door umbilical in the A post, all appeared clean and intact. I gave a spray with WD40 anyway.

The warning popped up, then went off. I'm unconvinced !

After looking at the previous posts about the difficulty rating I may just leave it at that ...

Are you able to lock the car with the door "open" ?

On the Fusion, with a front door pretending to be open, it could not be locked.

Yea, you just have to push the handle in from inside the car. 

2008 Fiesta 1.6 tdci twin cam 80K. Locking but then unlocking so I could not lock the car. I disconnected the signal wire to drivers door  and now the door locks manually as do the other doors. Oddly I can unlock 3 doors remotely with key but have to lock them manually. If only me and wife using car I manually unlock drivers door and reach over to passenger door the rear doors staying locked. You need a not overly expensive Multimeter.

The Multimeter. These will not load a circuit and will sometimes fall short doing continuity tests. In these instances I use a test light.

Black coffee helps white coffe may not work as well.

1 hour ago, frund said:

The Multimeter. These will not load a circuit and will sometimes fall short doing continuity tests. In these instances I use a test light.

Yes, popular misconception.