Valve Cover Harness
The 7.3 power stroke is a great engine but has a couple of faults. One of the faults is the valve cover harness. What happens is the harness will disconnect itself where it plugs into the valve cover. There are a few different repairs for this some people use a quarter or a zip tie, and there is also an aftermarket valve cover gasket that eliminates the connector. This repair doesn't cost very much unless it is misdiagnosed.