ironically, the name 'smart valve' turned out to be not so smart. it was the forerunner of the contol board and was discontinued. diagnostics on these systems is difficult and sometimes it's necessary to replace all components involved (ignitor, valve, wire harness and flame sensor) to ensure proper operation. if it has a smart valve, the furnace has some age on it. make sure you have a good heat exchanger before the customer ends up spending a lot of money on this repair. replacement may be an option to present.