A shower delivering only lukewarm water when you have set the handle to hot is typically caused by an improperly adjusted rotational limit stop (anti-scald ring) behind the handle trim that physically restricts how far the handle can pivot. Another common cause is a worn-out pressure-balancing mixing valve cartridge where the internal spool is stuck, preventing the hot water port from opening fully. Verification: Remove the handle trim and adjust the plastic limit stop ring to allow a wider rotation arc toward hot.
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