These Honeywell/Resideo WV/WT8840 series valves can be problematic. The older ones had an all plastic thermowell which would get old, craze, start leaking water, and then the water would damage the dual NTC thermistor that senses the tank temperature and cause an error code 5 which disables the main burner for safety since the control can't be sure of what the actual temperature is. That's why it's a dual thermistor and not a single one as usual, it compares the 2 and if they don't closely match it faults out.

That's what happened to mine, the WV8840B1059. I had the same barely lukewarm water issue and a code 5 sensor failure along with eventually a damp spot below the control where the water was occasionally dripping. I replaced it with the WT8840B1500 which has the metal cap on the thermowell to protect it and hopefully stop it from ever leaking.

You said that yours already has the 1500 on it, if anything is wrong it should be blinking a trouble code.

Oh and do also check your anode rod, it has nothing to do with it not heating but it is extremely important, I picked up a free water heater that was identical to mine to practice the gas control removal on before I did my own one. It had rusted out and leaked because the anode rod was corroded away entirely, the best part, it was 2 years newer than mine and yet mine is still fine because the anode rod is only about 50% gone (I removed and checked it while the tank was drained), I guess I have lower TDS water or something.