That's not an error, the game has booted and is in audit mode. Leave the coin door open, power off then on quickly, game should come up in attract mode.

Sounds like it is in attract mode and you're seeing it alternate between the high score and no score. System 3-6 games show all zeros or all the leading zeros on the score displays. You should see the playfield lamps lighting up and cycling unless the fuse for the lamps is blown. Add credits and try to start a game, but you should probably do some housekeeping before blindly starting it up in case something is shorted and locked on. And the 2.5A SB fuse for the coils is usually blown on games that haven't been maintained so starting a game may result with not much happening as the ball won't kick to the shooter lane.

Now comes the fun part. You need to find the reason for the 2.5A SB fuse blowing. Time to study Pinwiki, other threads, and other repair sites. You need to do the bullet proofing steps such as replacing the 40 pin header and sockets.

Look for a shorted transistor on the driver board. Less common is a diode shorted on a coil (it probably isn't this). The slings and pops are not CPU controlled and if one's switch is stuck it will lock on and blow the fuse. Sometimes the caps on the switches for the pops and slings go bad and short the switch and lock on the coil.

I've also seen blanking issues where when you power on the game all the coils fire at once and blow the fuse. This can be harder to diagnose as there's multiple things that can cause it.

There's a lot of things to look over and take care of. You've just got to read, dive in, and do it.