Noticed the temp in my '89 marquis spa seemed a bit low. I turned up the thermostat and didn't hear the relay kick in. I opened up the box and started checking continuity and finally decided the heater contactor was bad. I pulled it out and sure enough, the transfmer wires had burnt and opened up. So I ordered a replacement. Question...........other than just wearing out, what else would have caused this? It has a 110V coil and to my way of thinking, it's going to pull whatever current the transformer wiring resistance requires. If the coil wire contacts got oxidized, this would have driven the resistance up and this would have resulted in less current through the coil, so that wouldn't cause the coil to open. Any ideas?

Man that thing is a 1989 model. For a spa, it's old. For a contactor in real service, it's absolutely ancient.

Replace it with confidence.