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hi i am dan i can help you with this. first i need a little more information. what are you working on? what is the problem?
ok what is the model number of the heater? what have you replaced?
ok what is the model of the control valve? does the combustion blower run?
ok and have you checked the pressure switch?
it will be a switch with 2 wires and a small 1/4" tube connected to it. looks like a hockey puck. make sure that hose it completely clear and the vent pipe is not restricted in any way
it will be up near the blower
ok do you have an electrical meter?
ok how old is the heater? have you tried cleaning the flame sensor?
yes it should be shinny metal. doesnt have to be perfect but good enough. let me know how you make out after that
the flame sensor is at the pilot light. cleaning it requires removing the whole burner and pilot assembly
ok if it is all clean re asemble it and check it now
should be within 30 seconds
ok make sure the flame sensor wire is also making good contact to the gas valve
ok the flame sensor may be bad. the only way to confirm that is with a meter to read th mv signal from the sensor. if you want to try replacing it anyway would not be a bad idea since that is the most likely problem at this point.