Powerstar AE115. Neon light off - Water not hot.

http://waterheatertimer.org/pdf/Bosch-AE115-AE125_manual.pdf
Bottom page 10 troubleshoot

Add tempering tank to pre-heat cold incoming water
http://waterheatertimer.org/Tempering-tank.html

Call service technician

Delime heat exchanger.
http://waterheatertimer.org/How-to-delime-tankless-water-heater.html

Replace with tank type?
http://waterheatertimer.org/How-to-install-electric-water-heater.html
Gene

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

make sure you have the water connections right. the inlet goes on the blue side. Also, your pressure must be a minimum of .8 psi. If not there will be no hot water and the neon will be off. other possible causes might be a crossover in your plumbing or a stuck flow transducer. Here is a link to a guide to explain how to check these items. http://efficientelectricwaterheaters.com/Tech-Support/PowerStar-Tech-Support.asp

Water leaking into elements. Bad PC board. Undersized breakers. Undersized main panel.
Bosch Powerstar is a computer that happens to heat water.
If your 5-year-old laptop computer was malfunctioning, you would not be surprised.
But for some reason (false marketing by manufacturers?), folks believe a tankless computer is going to last 20 years.
Moreover folks also believe tankless will provide endless hot water while saving money.
The whole idea of tankless saving money is a load of sales-BS that defies physics and common sense.
There are no service manuals... except bosch website has troubleshoot section in product manual.
Call a bosch-qualified service tech.
Go to Lowes and buy ordinary tank heater, and fix your heater today for only $250.
Tank-type electric water heater can be easily repaired and maintained by homeowner to last forever... that is a fact
http://waterheatertimer.org/How-to-maintain-water-heater.html

http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

Gene

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

The left canister probably has a grounded element in it, which will keep power going after the water is off. Either that or you have a bad triac on your control unit

Product manual shows electro-mechanical thermal cutout page 9 but offers no troubleshoot steps except to say that overheating is cause.
http://waterheatertimer.org/pdf/Bosch-AE115-AE125_manual.pdf

http://www.bosch-climate.us/files/201009211859150.EP-23_Removing_and_replacing_element_assembly.pdf

Test elements:
Remove both wires off element.
Set multimeter to read ohms.
Watt and volt rating appear on side of each element.
Ohm = Volts squared divided by wattage.
For example:
Assume 240Volt circuit (240 Volts squared = 57600)
Assume 9000 watt element > check side of element for rating.
So 57600 divided by 9000 watt element = 6.4 ohms
http://waterheatertimer.org/images/ohm-reading-for-emements.jpg

Have you de-limed this unit?
Is there a check valve causing closed system in the plumbing, that might also cause decreased water flow?
Are there reduced-flow faucets or clogged aerators?
What happens when suspect cut-out element is no longer connected? The unit should continue heating.

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

Question posted under Bosch tankless.
Question says 'tank'
Brand, model, and type heater unknown.
Elements mean electric.
'Thermal cutout pops when taking shower produces cold water' indicates tankless, since tank-type would maintain some hot water.
No error code might indicate tank-type.
Add a comment with type, brand and model number of heater for possible assistance.

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

You've done the required first-step troubleshooting and maintenance.
If thermal cutout keeps tripping, and unit is actually reaching 185, then something is wrong with wiring, temperature sensor, gas valve, or PC board. Or combination of factors.
Call service technician, and open following link for some Bosch tankless resources including Bosch contact information:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

I don't have specific suggestion for tankless tripping breaker or if that is a circuit breaker problem or a tankless problem.

1) Move wires to another same-size breaker and see if problem persists.

2) Compare amp draw on tankless label with wire size and breaker size amp rating.
For example 30 amp breaker using 10ga wire is rated for 80% of listed amps or 24 amps.

3) If you are drawing full 30 amps, then 30 amp breaker is getting hot. Breaker trips with heat. Older breakers can trip more easily.
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

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

5) Be sure to read manual for yearly maintenance requirements.

6) Add a tempering tank for incoming cold water problem.
http://waterheatertimer.org/Tempering-tank.html

7) All gas tankless brands can be repaired by homeowner for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Call BOSCH 800-798-8161 tech support. If it is not too old even out of warranty they will help diagnose. You can get parts for free in warranty and buy parts at www.houseneeds.com out of warranty. If it gets hot fast and then stops, one of the "Thermal cutout switches" is cutting out. The water flow it probably blocked. (Just a guess) On many models they have a "Screen" and or a "Gromet" that you can clean out. If your water is really hard the whole unit may be clogged. I believe the "SAFE" thing to do is replace it but again I say CALL BOSCH TECH SUPPORT. I have also heard of people "Cleaning the whole units with vinigar, like cleaning a coffee maker. TURN OFF POWER! TURN OFF H20! REMOVE UNIT, then clean.

It is difficult to diagnose tankless water heaters remotely since there are so many variables to take into account. It might be helpful to check the troubleshooting information on your model at boschhotwater to see if a handy solution is available before calling out a service tech. There are PDF's viewable with more detailed information on troubleshooting (flow rate, power supply, inlet temp, graphics, etc.) and they have a tech support phone # which is very helpful.
If the light is on and you still don't have hot water, I would start by checking the thermal cutouts (breakers) on top of each heating module that could be tripped and need reset.

I can attempt to post walk-through if needed.