The problem is the stack over temp sensor located on the front of the fume exaust fan. The stack over temp sensor is prematurely tripping thus dropping the 24v to gas valve. The unit tries to re-light 3 times and after 3 failed attemps the circuit board goes into fault mode with the flash code of 1. This is the code for ignition falure. When this occurs the power to the furnace must be cycled to return to service. You can test if the stack sensor is bad by bypassing it. If the furnace runs normally then you know it's the stack over temp sensor. P/N B1370254 for the GMNT080-4.

Guys,, this thread started a year and 1/2 ago,, last add on was 2 months ago,, its time to move on

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Originally Posted by handyone

I just resolved this problem with my own furnance. I had the same symptoms and it did in fact turn out to be the flame sensor.

Two screws and a piece of sandpaper was all it took. Remove the sensor and clean it before you pay for a service call. What can it hurt to try?

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I just wanted to say thanks, you probably saved me $100 for 3 minutes of easy work even for a notsohandy1

Bingo!! Hold your cards. I had the very same problem, intermittent furnace lock out. When I had a HVAC tech out he found no problems although he did change one setting on the Honeywell VisionPro IAQ thermostat which really didn't have anything to do with the problem. By coincidence the furnace worked fine for awhile and then the same problem, furnace lockout. Thanks to NorthernHeat's suggestion I removed the condensate drain hose even though the trap seamed clear and full of water. About 6 oz of water came out indicating a blocked hose. The hose was NOT blocked but it had drooped below the level of the drain hose exit hole and had trapped water causing water to drain slowly. The slow drain caused the intermittent problem. I placed a shim below the point of the droop so that the drain hose runs a straight line to the trap. Problem solved. Thanks NortherHeat!