Likely not the coil.

Are the opto transmitters amber or clear? If they are original probably need to be replaced. Could also be connectors on the board. Honestly it’s less hassle to just replace the boards entirely due to age.

You’ll want them looking like these:

F16DA8DF-2C0A-436C-AD3E-EF92DA33BD70 (resized).jpeg

Trough optos! Very very common problem!

You want QED123 transmitters and QSD124 receivers. I buy them from Mouser, but Pinball Life sells an emitter and receiver LED that works well.

If replacing receivers, check out this thread:

https://pinside.com/pinball/forum/topic/how-to-determine-the-c-e-of-black-opto-receiver-leds

My Demolition man has an aftermarket trough opto board on it, but if your transmitter board has big resistors soldered to it, those resistors can be open (not usually intermittently... ), or have broken solder connections. (VERY common).

Also the connectors to both the receiver and transmitter trough opto boards can have broken solder connections. Reflow the solder at the connectors if necessary.