There is hot water in the sink in the bathroom but none in the tub/shower . Hot water is fine everywhere else . Someone said it could be a cartridge?

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Someone said it could be a cartridge?

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and they were correct.
Sound like you have some debris blocking the hot water inlet port on your cartridge.
Shut the water off at the house shut off and open up the valve. Remove the cartridge and check and clean the inlet ports. Now turn the water on for a moment to flush out the supply. Reassemble and test. Better now? Good luck, Tom