As soon as you turned this plus4 on, it just displays MONITOR.
If you get anything like this, grab a cartridge
and plug it in. If the cartridge works, you have a ROM fault.
You should always replace the capacitors in this old kit before they go out of specification.
said caps, it only took one chip swap to find the faulty silicon.
Ebay sourced replacement U19.
I had already tested it against my working plus4's PLA before I bought this one!