Yardman Riding Lawnmower problem

The problem you describe is most likely caused by the operator presence system. The most likely culprit is the seat switch. You can test the switch in the following manor. Remove the wires from the switch at the plug. than, using a small piece of wire, jump across the two wires or connectors at the plug on the side coming from the engine, not the switch itself. Then restart tractor and engage PTO to see if the problem is rectified. If so, simply purchase a new switch (they are very inexpensive) at a local small engine repair shop. The switch may be bypassed but it is not recommended and it is dangerous.