Hello, I have a 1991 Rheem 40gal W/H Model 21VP40-1 which decided to not heat water. The Power Vent is on (exhaust) and the electric Pilot Ignitor cycles on and off because there is no ignition of the gas. It seems to me that there is no gas to ignite as I believe the valve is not turning on. (No smell anyway). There is gas to the heater and the valve is turned on. How do I determine whether the problem is a faulty Gas Valve, Ignition Control Module or other?