Hi: We have a Sears furnace model #NTGM075EGA3. It is from 1999. Every 15 minutes the burners shut off. The fan continues. We noticed the relay switch popped so we reset it and it runs for 15 minutes then shuts off again. Do you know what could be the problem?
Sandi

Here is a parts diagram of your furnace. Which part is it.

ICP | Model #NTGM075EGA3 | GAS FURNACE | PAGE 1 | SearsPartsDirect.com (http://www.searspartsdirect.com/partsdirect/getSubComp.pd?modelNumber=NTGM075EGA3&productCategoryId=0912130&brandId=0492&modelName=GAS-FURNACE&diagramPageId=00001&documentId=P9070097)

Here is a parts diagram of your furnace. Which part is it.

ICP | Model #NTGM075EGA3 | GAS FURNACE | PAGE 1 | SearsPartsDirect.com (http://www.searspartsdirect.com/partsdirect/getSubComp.pd?modelNumber=NTGM075EGA3&productCategoryId=0912130&brandId=0492&modelName=GAS-FURNACE&diagramPageId=00001&documentId=P9070097)

It looks like it could be between number 17 or 18 part number Z

Could be a weak switch part number 18

Or the exhaust might be partially restricted and I would check this first before replacing parts. Ice and snow or other items outside migh have it restricted.

Gasket, limit switch is part number Z
If you replace the limit switch make sure you get a new gasket

I looked up the exhaust and didn't see anything - can I poke something in there to be sure?

Usually the exhaust is disconnected for inspection but I guess you could poke around if you want.