I have a Carrier/Payne Heater that is 20+ years old. It was working fine but now when it calls for heat, the pilot light turns on and after a few seconds it will lower the flame and then it will start clicking again to turn on the pilot. The gas valve starts making some noise but the heater never turns on. I replaced the gas valve 646AX but the problem continues.

Sounds like you may need to clean the flame rod, or the piolit assembley