The furnace does not fire up even though I installed new sensors( flame & Ignitor). The gas valve opens and I can hear the trigger switch sound ("click"), the furnace repeats this cycle four times and then stops. During the cycle, the red & green lights on the ignitor control board are blinking simustaneously, but after the cycle is over, these lights blink alternatively like twice per second.

How can I fix it?

Thanks,

Frank.

PS. The furnace uses natural gas.

Somewhere on the furnace (often inside the blower compartment door) should be a key to the blink code. Nearly every manufacturer uses different codes for a given problem.