Unit is about 8 years old, cools but doesn't heat. New thermostat, new plenum, gas lines not clogged, burners clean, air filter new, igniter working but gas valve won't open? Circuit board flame light will come on for 1 second then goes out? Green light says system OK then the flame light stays on for 1 second then goes out and stays out. Maybe gas valve needs cleaned? Running out of ideas

Possibly the gas valve needs cleaned. Or the ingnition module is bad and not sending a signal to the main gas valve to open. Could be many things.

Aaron