If it is from 2001 it probably is a moen pressure balanced cartridge, any chance that the temp limit stop was not put back to the original position?
I don't think that the cartridge is backwards since the OP said that is was good til half way mixed.
I have seen bad or stuck PB spools.
Please keep us posted.
Is the diameter of your cartridge the size of a cheap hot dog or a deluxe polish sausage. This may give us an idea of your valve body.
Easy on the big $15 paycheck Rockefeller, wouldn't want to offend anyone. If I'm giving advice, I would've done it for free. Pick up the phone and pay someone to fix it.
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Replace the new but defective cartridge.
I would say chances are a peice of rubber from the old cartridge is stuck in the hot side inlet, that is the most common diy mistake i have ever seen.