The heat control valve appears to have stuck, the water is only luke warm

is the water temperature dial turned up? or even functioning?

First thing is to know when it was installed? Check the top of the unit and see if it has in and out marks on it. If it does are they correctly hooked up? Installing them backwards causes this.

Maybe the problem is bad faucet cartridge.
Test for crossover:
http://waterheatertimer.org/Crossover.html

Tankless are really made to be thrown away when problems start.
It's like replacing parts on your computer ... for real, when the hard drive crashes and new one is installed, then rarely are problems repaired satisfactorily.
Repair any brand tankless, any problem, permanently, same day, for $385
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

check your expansion vessel is not flat.if so recharge..simple way to check press in valve on vessel if water comes out or little air recharge..if water still in vessel the diaphram in vessel has split so replace vessel?hot water relys on expansion??

If all the other faucets work, then the problem is the mixing valve in your shower. You will need a new shower fixture.

the dip rod in heater may have failed. did you see any white chips in sediment while draining. if so its the dip rod, they do fail sometimes. If the water heater in your home was built between 1993 and 1996 and you are beginning to find small pieces of plastic in your faucet aerators, shower nozzles and washing machine supply tube, or if your water temperature is suddenly not as warm as it used to be, the source of the problem may be your water heater's dip tube. The purpose of a dip tube is to carry cold water to the bottom of the tank, where it is heated for use. The defective dip tubes tend to crumble and disintegrate into the tank. Over time, its plastic ! have a very good day !

Water heater is heating water but only luke warm water arrives at faucet.
Open TP valve and confirm that hot water is available.
If water at TP valve is hot, then problem might be plumbing-related, for example crossover.
http://waterheatertimer.org/Crossover.html

Before calling service technician, open following link for Bosch tankless troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Also check that by-pass valve(s) is/are, in proper position at back of tank.