First of all, I am new to pinball machines entirely and just helping out some family. But I do have some understanding of electronics in general.

Symptoms:
System turns on but displays 04 00 code in bottom left display.
Can get into attract mode by holding down the right-most button on the inside swing out door several times.
Can add credits by bumping the switch contacts on one of the coin slots.

At this point I cannot get the game to start though. I am just pressing the button on the left side of the front panel. Is that correct?

Attempted fixes:
I did try to re-seat the 40 pin connector, no change.
The batteries in the unit had failed and corroded. I removed the batteries and cleaned the contacts. No acid had gotten on the main board. Batteries then replaced (positive side of each battery aimed to right hand side as per board marking). Did not change the 04 00 code though.

Once it is in attract mode the lighting and sound both seem functional.

Any suggestions on what I should be looking at?

Thank you

Update:
I ran the diagnostics and two switches showed up - 18 and 34.
Bumped 18 switch and was able to "unstick it".
Balls will not bump into the shooter lane. I can manually bump them by actuating the lever from below. Also, after the balls drain they will not move to the right side where they would load into the shooter (if that worked).

Lighting also seems intermittent and it is not clear what is causing this. Are there any common areas I should be looking at?

Is it likely the solenoids have failed? How are these replaced?

Thanks