I have a five year old Sterling unit heater m#RF-45 which uses a Honeywell Smartvalve M# SV9540M 2229. On call for heat HSI energizes and lights pilot but main valve will not open. I have cleaned and inspected HSI assembly and all looks well. Ventor motor and main blower work fine. On one occassion I jiggled wires from HSI to plug on valve and valve opened, but only on the one occasion. I am also only getting 13 volts to one side of rollout and limit safeties. This is the first time I've encountered this type of valve which I do not want to replace until I'm sure it's faulty as it is $850.00 list ! Any help would be greatly appreciated. Thankyou.