Generally speaking, the IBTS ought not drop below the BT reading if the IR sensor is clean. I’m guessing you did, but did you clean the IR sensor at the same time you cleaned everything else? The recommendation is to clean the IR sensor using alcohol which is not very effective for cleaning roasting ‘stuff’ (probably mostly smoke residue) from the IR sensor lens. Takes a few applications and some light rubbing each time. And there is no feedback: I’ve never seen anything on the cotton swab. So I concluded the contamination must be the same color as the cotton and that probably means something left behind by smoke. But I’ve never cleaned the sensor without seeing an upward shift in IBTS readings.
The air inlet for the IBTS cooling fan is at the base of the Control Panel pedestal. It’s only a guess as I’ve never seen it happen but I assume the smoke is entering there at the base and originates from the area of the drum door when the door is opened to drop the beans… ?? In my case the opportunity is always there for the contamination as I roast pretty dark and there’s a fair amount of smoke at the exhaust by the time I get to IBTS = 449°F.
Bruce