I installed a new water valve in my bosch125x ,the unit lights fine but the water doesnt heat up above luke warm any ideas?

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

1) Add tempering tank to preheat incoming cold water.
http://waterheatertimer.org/Tempering-tank.html

2) Read product manual or call Bosch to make sure unit works with pre-heated incoming water. Some incoming water sensors will not trigger combustion is incoming water exceeds certain limit.

2a) Read product manual for required yearly/monthy maintenance.
Train wife to delime tankless yearly.

3) Install bigger gas meter.

4) Solve entire moneypit problem today for $325
http://waterheatertimer.org/How-to-install-gas-water-heater.html

5) Don't install tankless unless you have a lot of money and time for headaches.
Don't believe promotional literature. Tankless promotion is BS for average do-it-yourself folks who want to save a buck on water heating. Estimated yearly saving of $100 means Tankless do not save money long term. All costs are energy: Purchase price and installation negates several years saving up front. One repair will negate another 5-8 years energy savings. By then you need new unit.

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??

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

Check cylinder stat , if fitted check 3 way valve is fully open on hw side . Check water level in header tank

OVERHEAT STAT OR FAN

vailant boilers are one of the best on the market and should last you more than 6 years depending on installation and servicing.
the engineer you had should not have replaced the diverter valve if the boiler was firing up when the taps are opened. if you was getting luke warm water with the boiler alight it is a classic symptom of a blocked plate heat exchanger on the central heating side.
a new heat exchanger can cost around £100 depending on the age and model of the boiler.alteranatively it could be descaled but may clogg again. it would be a good ideal to fit a magna clean filter on the system to help prevent further debris if you have a dirty system .if you don't trust your engineer i suggest you ask another. find out if he is registered first and don't pay until you are satisfied with the work. if you are asking the same engineer back try and get a discount off the work as he has failed to rectifiy the original fault and fitted the wrong part due to wrong diagnostics. if he is registered you can complain to the governing body ie corgi or gas safe register.
good luck.

Some circulation pumps have a 3 phase mixing valve (cold,warm,hot) to regulate the water pressure in the building; no way to know unless you trace the plumbing. Basically, the hot water temperature could be lost because the circ-pump is cooling it with the cold water supplied to it. This is used to balance the pressure (aka Pressure Balancing Circ-Pump), so heater is really working overtime to heat the water. If the pump has two in and one out that's more than likely your problem; but you should consult a "old-school" plumber in your area.