Amana 90% model #GUCAO45AX30 SER# 0109143635 UNIT MAIN BURNERS LIGHT UP THEN SHUT DOWN IN A FEW SECONDS. DRAIN LINES ARE CLEAR, VENT PIPE CLEAR OF CONDENSATION. I BYPASSED T-STAT JUMPERED R-W NO HELP, INSTALLED NEW FLAME SENSOR STILL SHUTTING DOWN. UNIT HAS TWO PRESSURE SWITCHS OFF INDUCED DRAFT MOTOR, HOSES GOOD CONDITION AND TIGHT. I HAVE HAD PROBLEM WITH WATER IN 2 INCH ABS VENT PIPE, NOT SLOPING BACK TO UNIT, THIS ONE SEEMS TO BE CLEAR. SHOULD I CUT PIPE AT FURNACE AND TRY TO START ?.. MAYBE IN THE CIRCUIT BOARD ?

Check to see if the flame rod tip is in the flame(like the hot junction on a thermocouple). the pressure switches are a normally closed switch and a blocked flue could cause them to open. the circuit board should run through the safety switches and check them before the gas is opened at the valve. also the drain lines that you see at the inducer fan should be free of condensation but if the inducer fan has condensation in it, it will open the airflow safety switch. you will hear the crackling noise from the fan. also there should be a trap below the fan that should be full of water to keep the fan from pulling air from the wrong place. I would not suspect the board at this point