Run hot water at a nearby sink. If the sink gets hot and the shower does not, the water heater is probably producing hot water.
If only the shower is affected, focus on the mixing valve
A worn or sticking shower cartridge can restrict the hot side. Many valves also have an anti-scald rotational limit that can be set too conservatively.
Ask whether the problem started after plumbing work
A newly installed cartridge, a partially closed stop, or a changed limit setting can explain a problem that began immediately after service.
If the whole home lacks hot water, move upstream
Check whether the water heater has power, whether it displays an error code, and whether other gas appliances operate when relevant. Do not open gas compartments or high-voltage covers.
Separate temperature from flow
A very restrictive showerhead or a tankless heater can behave differently at unusual flow rates. If water is only lukewarm, note whether temperature changes when flow is adjusted slightly.
Know when to call
Get qualified service if no fixture has hot water, you smell gas, the heater repeatedly faults, there is leakage, or the shower valve repair requires opening finished walls or pressurized plumbing.
The shower-versus-sink comparison is the highest-value test because it tells you whether to investigate the bathroom fixture or the water heater.
Sources
- U.S. Department of Energy — Water Heating — Water-heating efficiency, temperature, equipment, and household hot-water use.