FURNACE Problem...Any Suggestions - GR, Michigan

[Jack Nguyen]

The FURNACE on one side of my duplex stop working 6 days ago.  Call the service guy, he replaced a switch and work for a day then the same problem come back again. I call the service guy back, he replaced a different thing and the furnace is working now. So far 2 times service he charged me $400... He advised me to replace the FURNACE which cost about $2400 or replace the HEAT EXCHANGER because it's might have a small crack on it...to me the furnace is not look too too old at all, it's 90% effency. ((((So, should I replace the FURNACE or HEAT EXCHANGER?)))) Thanks