A shower failing to blend hot and cold water properly—resulting in extreme temperature swings or exclusively lukewarm flow—is typically caused by a faulty, mineral-scaled pressure-balancing cartridge spool inside the mixing valve. When daily wear or hard water deposits prevent the internal spool from sliding freely to balance incoming water pressures, the mixing mechanism fails. Verification: Shut off the main water supply, pull out the mixing cartridge, clean or replace it, and test handle movement.
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