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I have a Taco 006-ZB4-4 circulating heater. It gets too hot to touch and doesn't seem to be pumping. No noise and pipes are not hot beyond it. I removed it to see if it was maybe bound up but seems to turn easily. Can I get away with replacing just the cartridge or are the coils no good? How can I test it? I don't want to buy a cartridge then find out I need a whole pump.