Needed to upgrade a bathroom. Took the room down to the studs, rewired the electric, added tile walls and painted. New shower Koehler head and knobs. Everything looks good but shower's "hot water" is only tepid. Temperature selector turned all the way to hot side. Rest of the house has normal hot water. What would cause a new shower to not have hot water only tepid water? Mystery to me. Any thoughts appreciated.

First place to look is the mixing valve. I don't have Kohler products but use Moen. the same issue popped up and it was a bad gasket inside the mixer that was the culprit.

The valve is backwards. Pop it out, turn it around, and reinstall it.

Or it’s not adjusted properly. This video might help: Link.

I had this problem on our showers in our new house. Turns out the valves had a scald safety block. I took them out. Problem solved.

Pop out the valve cartridge and adjust the mixer. They come set on lukewarm

Dont pop out the cartridge. Remove the handle and a black circular piece, the handle stop, pulls out. Rotate it so the handle moves further to the hot side when reinstalled.

I call Frank the plumber, ask him to deal with it, then pay him.

Used to do it all myself. Built several houses, remodeled a few others. Now I'd rather just call Frank, and I usually only have to pay him once per problem.

There is an anti-scald device built into the valve. It can be adjusted. This is very likely your problem.

You need to take the handle and then cover the plate off. Look at the valve where the knob comes out that the handle attaches to. There should be a clear round plastic sheet and behind it a red(may be a different color) piece you can pull off and turn then put back on to adjust how hot the water can get. It should say hotter and colder or something simular to show which way to go with the adjuster.

If your diverter has valves on it check to see if they are open all the way.

Did you solder the diverter in? If so was the cartridge left in? If you left it in the heat may have damaged the cartridge.

Pull the cartridge out. The may have little rubber gaskets that may have unseated causing the flow of hot water to decrease.