cold shower

I'm having a problem with my moen posi temp shower valve. So I have a tankless water heater, im in the process of fixing the elements right now so i can't actually test for hot water, but I can turn the water back on. I replaced the shower valve as it was giving me issues. Here's the problem...

i have a bathroom with a shower that about 3 years ago no longer gets warm. i replaced the diverter cartridge, and it is exactly the same. i played with the supply valves and it doesnt get warm unless i turn the cold supply completely off. since i replaced the diverter cartridge, it...