Hi. I have a Payne brand central heat / cool system and the blower has recently stuck on. If I bang on the lower section of the housing I can make the blower turn off but it just sticks back on on the next cycle. After dismantling and some amateur diagnosis I located an arcing relay (KUP 2D17 - the HFR) and changed it out for a new one. However the new relay immediately did the same thing - arcing and sticking on.
Why would this start happening when it has worked fine for years? Is there a larger problem here? Thoughts include a faulty blower motor overstressing the relay contacts, or a failed suppressor capacitor across motor terminals?

Do any of you have previous experience with this sort of issue? Is this something that could be relatively easily addressed by any technician, or do I need to seek out someone specialized in my particular brand of equipment? What should I expect as a 'rough order of magnitude' cost to have this fixed by a pro?

For your reference, the front panel of the furnace lists the following:
Model No. 518A042CC
Product No. 518A00042000MAAA
Serial No. 1181C88695
BDP Company, Division of Carrier Corporation, Indianapolis, IN. City Of Industry, CA Rev C 300917 2018A

Furthermore, an ID panel mounted in the burner area, immediately below the Gas Supply controller identifies it as:
Model: 394GAW048060
Series: 394GAW048060AAEA
Serial: 1281C92566