I have a Goodman Model# GMP075-3 Serial #9501043865 My furnace heat up for about 15 minutes and then goes out. The HVAC Tech replaced the flame sensor and the auxiliary limit switch and that did not resolve the issue, The Tech says that he checked everything he could think of and the heat exchange may be cracked and I should replace the furnace.

Before replacing the furnace I wanted to know if there may be something else that can be checked that he may have overlooked a new furnace is very costly at this time and if I could possible get it to work for a month or two that would be much better financially for me. Any thought would be greatly appreciated

This is a complete Goodman service manual for your furnace as well as a few others. Make sure you are looking at your model numbers info when using it. It is full of trouble shooting charts. This will keep you busy for awhile. The problem you have could be caused by a few different things and this addresses just about all of them.

http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf