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cfw-
The only filters on mine are the main furnace airflow filters, and they're fine. They sit outside the unit where the return airflow comes in. Mine is an upflow style unit.
However, I did discover that the high temp limit switch (sensor) might have been worth checking out. It is in the internal airflow (near the heat exchangers). I made a little jumper to "go around" it, and for a few days I thought it was doing the trick, but no, eventually (within two days), it stopped coming on again. But I turned her off, and back on, and she fired right up and ran for eight days before failing again.
CFW, if your unit has a diagnostic LED, maybe it's a Plus 90i model. That's a generation newer than mine. Mine has a one speed AC motor for the main blower, and the control electronics are run from a 24 volt AC transformer tap. No microprocessors. How large was this "filter" you're referring to, and whereabouts was it located?
I still am leaning to the idea that it's the redundant gas valve (it's the only part that hasn't been replaced - unless the sevice tech or I missed something). There are two servos or relay operated valves on it, one for the pilot and one for the main gas burner. It's either that or some really obscure electrical connection problem, corroded or loose connection.
I was trying to get a scanner to send images from my owner's manual over to HayZee518, but that turned out to be a no-go, so I'll probably have to go to Kinko's and scan the schematic so I can send it. We'll see. It's been mild out here in Michigan up 'til now, but that can't last forever!
Thanks for your input, though. I'll keep you posted!
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