I'm tearing my hair out on this one guys.
2004 XJ8 X350 N/A
DIagnosed a restricted performance issue back to the fuel pump control module which is internal to the rear ECU on the right side of the trunk and it's a Visteon unit.
I bought a used unit with completely matching numbers (except serial number of course)
I disconnected the battery and fitted the doner unit in, no problem.
But on attaching the battery to be able to program the unit to my car using SDD, the alarm goes off and can't be turned off no matter what I do, iv tried every trick out there with the remote/door locks/ignition etc etc but I have to disconnect the battery again to stop it. How on earth can you fit a doner module to a car if you can't turn off the alarm? Has anyone else had to do this?
Cheers

remove or destroy the sounder then go program the REM 🤷‍♂️🤷‍♂️🤷‍♂️🤷‍♂️

I can't help much but I had a problem recently when an alarm went off under the dash.
The only way I could stop it was when I removed both connections on the battery and touched them together.
I scanned the car and got a U code which was something connected with an unexpected message on the CAN system.
I am guessing that it was connected with a dodgy status switch in the driver's door actuator.
It has only happened once and I had no idea about the sounder under the dash.
A couple of things to think about:

Scan your vehicle for codes

Also

Dismantle the failed module and look for:

Moisture ingress
A failed component eg overheated resistor or condenser
Pins not making contact
Dry joints
Inspect using a magnifying glass
You never know you might be able to repair it

One other thing
There are people who will test ECU's on the bench

Can you not reprogram while in noisy mode?

I would but it will be about 30 mins of a very noisy alarm siren going off so i think it's easier and for my sanity to just take the wheel arch liner off and disconnect the siren. Until the doner rear ECU is programmed to the car.