GMP 100-3 furnace trouble light 1blink. I replaced flame sensor and gas valve. Igniter glows relights burners 3-5 seconds shuts gas off. On third cycle it locks out. I don't want to replace circuit until I get some suggestions.

Remove, clean, & tighten all grounds then reinstall. Remove the flame rod, clean with a scotch brite pad or fine steel wool & reinstall. Sometimes they are contaminated from the factory. I always clean one before installing.