Aquastar 125b ng: after 2 years of use water is not hot enough

1) If tankless is not showing error code, the problem might be single-handled faucet cartridge:
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

2) Catch up on maintenance, such as cleaning combustion parts, have gas pressure checked, etc.

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

Did you check to make sure the shut off handle is on the on position?

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

This should run two showers at once with no problem. If you have a shut off valve on the cold side of the heater, try closing about halfway. This will slow the water flow and the water will have more time to heat up as it pases through the tank. If you dont have a shut off valve, just cut back the hot water valve on your shower and do not use any cold water. If it gets too hot then turn on a little cold water. The object is to slow the speed of the water going through the tank. Tankless heater take a little getting use to but they work great when you know how to treat them. On my shower I turn the hot all the way on until it gets hot and then add a little cold water to bring the temp down. You may have too much water going through too fast to be able to heat it up to what you need. Try this and see how it works.

OVERHEAT STAT OR FAN

Does it happen to any other fixture? If not, it's most likely your shower valve and not heater.

The cold main water pressure is too low to maintain a constant flow. Make sure you have no other appliences on, or your not comprimised by the lowering of pressure

Aquastar 1600H NG Natural Gas Tankless Water Heater Bosch AquaStar Model 1600H NG Natural Gas Whole house tankless gas water heater, 117,000 Btu input, natural draft with hydro-generated ignition. Provides endless hot water for one major application at a time . Can replace 40-gallon storage tank water heater. Optional Accessory: Model AQ1- Power vent with built-in... More freeze protection when venting horizontally. Capacity: 1 to 2 Bathrooms Dimensions: 8.75D x 18.25 W x 29.75H Weight: 40 Max GPM @ 45F Rise: 4.2 Energy Factor: NG .80 LP .84 Minimum Input: 28,000 Maximum Input: 117,000 Vent Size: 5 Gas Connection: 3/4 Water Connections: 1/2 Warranty: 12-year warranty on heat exchanger, 2-year on parts Features: Endless hot water Can save up to 50% off utility costs annually Perfect for one major application at a time No standing pilot, hydro generated

I found the manufactures website and read this specific tech bulletin. I will be replacing or repairing my water value soon.

http://www.boschhotwater.com/Portals/7/TechBulletins/TWH-G1-25_Water_valve_rebuild_B.pdf

The bulletin said the following:
If the burners fail to go off immediately when the
water is turned off, usually due to mineralization in
the water valve on older heaters. To test for this
possiblilty, run hot water at a faucet, then shut off
the cold inlet supply and observe if the burners go
off immediately. If they do not the Aquastar will
overheat and shut down on safety depending on
the delay.