Did you check the flame pattern? This happened to me once. when I looked at the flame, one burner flame was different from the others, smaller flame. Removed manifold, found spider nest in that burner.
Looking for thoughts, found manual reset safety switch at burner box tripped about a month ago. Replaced plugged air filter and cleaned evap. coil.
Check HX all around, manifold, temp. rise .... ect. = all seemed fine.
Replaced all three safety switches proactive.
mod# T9MVP075F12A1 ser# Lo133 67579 = ICP with two stage gas, two stage inducer and seemed like ECM blower ( little different ).
Found safety switch tripped again, I'm thinking I'm missing a service bulletin or the HX is bad and I can't find the issue.
thanks dan
Did you check the flame pattern? This happened to me once. when I looked at the flame, one burner flame was different from the others, smaller flame. Removed manifold, found spider nest in that burner.