I have what appears to be a recurring problem with the trap door. This is the vertical pop up door that allows the ball to pass from the shooter lane directly to the gun. It opens when you choose to plunge from the gun, and also in certain multiballs to get the ball directly to the gun.
Occasionally, there will be a ball jam in the lower gun area. This will cause the game to think that the trap door is stuck closed, when it really isn't. This is not a big deal, as the ball jam will clear as the gun loading kicker will cycle through and clear the jam.
The problem is that the trap door stuck closed error does not clear. The error will appear at startup and the game will play, but not allow the trap door to be activated. You cannot choose to plunge from the gun, and balls will not load to the gun from the trough in multiball.
The only way that I have found to clear the error is to do a factory reset. Not the biggest deal in the world, but wipes out all the settings and scores. Does not happen often, but annoying. A google search finds a couple of old RGP threads where factory reset is the only answer given. Anyone have a better answer?