I would suggest pulling out your electrical diagrams and looking for the conveyor auto mode input. Verify the signal is indeed bad. Then examine the input connection to see if the wiring looks standard or aftermarket. If aftermarket then track what it is attached to. If standard, then check also what it is connected to and if it indeed is connected to the conveyor. If it does connect there, check for bad connection or bad relay. I believe the machine also has conveyor settings in the parameters that could have been changed so that it is looking for a signal that the conveyor is not providing. I would look closely in the conveyor box to see how the signal is routed. If you find no wiring for the signal then pursue the parameters.
Experience is what you get just after you needed it.