OP

Quoted from pinball_ric:

I have a comet I'm trying to bring back to life after it's sat for awhile. When I power it on it doesn't go into attract mode but the LED display on the board stays 0. If I push the diagnostic button on the MPU the game will boot into attract mode. There is some stuff on the player 1 display and the credit display which I thought was just garbage from the MPU partially booting. But the signal to keep the 555 from triggering is being generated so I believe the MPU is still running. Now I'm wondering if what is on the display is an error message. I forgot to check what it says before leaving for work this morning.
I do not currently have batteries in it. Previous batteries and holder were removed because they started to leak.