The heater is about 12 years old. When its turned on the fan comes on and after 60 seconds I can hear the gas valve click to open, which is followed immediately by the igniter start clicking to fire, but it does not light. It will cycle like this 3 times at which time I can smell the gas and then turn off. If I clean the filter and then try the heater it will light. The filter cartridges are never very dirty and it makes me wonder if the pressure sensors are failing. I want your opinion as what components should be replaced. Thanks for the help.