I HAVE A GOODMAN FURNACE THAT HAS BEEN ACTING UP AS OF A FEW DAYS AGO. I HAVE A PROGRAMMABLE THERMOSTAT AND WHEN I ASK FOR HEAT THE FOLLOWING OCCURS:the blower comes on about twenty seconds later the thermocoupler engages and the gas fires. The gas only stays on for a few seconds then shuts down. Immediately the whole process starts again. A troubleshooting test suggests it might be a pressure switch. I conducted a continuity test and the switch seems OK. I also checked the flu for blockage there is no blockage. Any help would be appreciated.:confused:

Clean flame sensor with steel wool or sand paper.