I have been working on a G27M5-120A-2 serial#6399b12127
the furnace will not stay on high fire. Low fire runs fine gas pressure is 1.70 inducer pressure is .70'w/c on low. Furnace trys to ramp up to high fire pressure goes up to .84'w/c then drops out second stage circuit board relays start to chatter. This all happens in about 10 seconds. I have changed out circuit board on second stage and gas valve and no luck. Has anyone had this problem thanks Grove