Glad you found the thermometer as I had no suggestions how to get it out of the pipe. As for the pump wearing out, no. There are no contacting parts in the pump case except for the shaft seal which is on the pressure side. The clearance between the impeller and chamber is close but does not touch so wear should not occur.
Are you getting bubbles out the returns? If so it's probably an air leak somewhere. All it takes is a very small leak on the suction side to mess things up. If the lid O-ring looks OK just try some silicone grease first. Easier and cheaper. To find the air leak you could use a squirt bottle to drip water around pipe joints and/or get a stethescope and use the open end of the hose to listen for air leaks around joints. Something I suggested a while back was to smear vaseline around pipe joints as that would probably seal a tiny leak temporarily so you can find it. Later wipe it off and use alcohol to remove any residue. Any thick grease will do. If you get it on any O-rings or non-plastic material, it may eventually attack the material which is why I said wipe it clean when done.
Hope this helps.
Al
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