12.55v on pin 3 is good. If you earth pin 4 and the light doesn't come on (switch plugged in) light in switch is stuffed.

I haven't pulled a switch apart, not sure if you can replace the bulb.

Someone on here told me that a workshop was replacing switches all the time. Now I can't see the contacts in the switch causing problems (between pin 1 and 2) but it could possible be a blown bulb in the switch that might be causing the issue.

When the "Sub tank" light is on in the dash the system will not work.

Have you tried disconnecting the battery and then try to access self diagnostic mode?