I had 2 parts replaced on my 20 yr old furnace dealing with the igniter switch. Now the furnace works for several cycles, then eventually, won't restart without manually flipping the switch on the side of the furnace, (which I guess is a reset switch) After flipping the switch, the igniter starts clicking and the furnace starts. This has gone on for a few months, sometimes going a couple days between flipping the switch. Now it won't restart and nothing happens when I repeatedly flip the switch... Help

Ensure that there is power to the furnace:
Try turning the fan to "ON" using the fan switch on the thermostat to test for power to furnace.
Check the circuit breakers at the electrical panel to make sure they are in the "On" position.
Check the SSU switch (it looks like a light switch on a gray box located at the furnace) to be sure it is in the "ON" position
The gas pipe for the goes into the gas valve is there a T there and a 6" pipe "drip leg" right below the Tee that supplies the gas so if there is condition in the gas line it drips to the 6" drip leg below the gas valve and its capped there it?

What 2 parts were replaced before?

It sounds like the furnace Igniter Sensor. This sensor senses when the ignition has lit or if it has not lit it turns off the fuel source. Pilot Igniter Sensor senses when the pilot igniter has lit and allows the continuous flow of fuel

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Try cleaning the flame sensor with steel wool,scotch brite pad,or sand paper.