M1MB090A Intermittently Working

[Dowayne]

My in-laws have a M1MB090A furnace - a/c that is just driving me crazy. Has given me problems for a while now. At first it was making ignitors bad and finally it stopped doing that. Then it broke down in the summer because the coils got too dirty for the a/c and we cleaned out the whole system. We have went through 3 thermostats over the last 5 or 6 years, which I guess isn't so bad considering. Now, I got it so it works fine and cycles correctly. It makes it to the correct temperature before it shuts down. It runs good for a day, almost 24 hours, and then it starts blowing cold air for a long period of time and shuts off by popping the main high limit switch. I haven't actually checked the temperature but it really doesn't seem hot enough to shut it down. I can reset the switch and if I wait about an hour or so, it seems like it just runs again for that 24 hours, but then does the same thing over again. I have tested the auxiallary and main high limit switch and got 0 ohms while off. I am just at a loss on how I can fix this. Can anyone help me, I would really appreciate it. Thanks in advance.

[Robert]

When it quits working, do you have 24vac at gas valve.



Thanks,
Robert

[Dowayne]

Just for clarity, are you meaning while it is blowing just cold air or right after it kicks the main limit switch off?

[Robert]

When blowing cold air. Also, can you see a burner flame during this time ?


Thanks,
Robert