I have a power star ae115 electric tankless water heater. It has performed flawlessly for approximately 5 years. A couple weeks ago it stopped producing HOT water but is still heating to Luke warm. The " reset switch " on top of the left canister, or the exit side, keeps tripping. It works fine after resetting until I stop running the water. A couple minutes later it trips again.

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

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

Call Bosch. And exercise your 10 year parts warranty rights.
Copy following link for Bosch power star manuals:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html#Ariston
Check circuit breakers, fully off and then fully back on. Test if voltage is at least 80% rating plate on unit. Check that temperature dial is set correctly. Call for service again.
Don't install tankless computer that heats water. You cannot fix this computer yourself, and they are problematic for owners.
Go to Lowes right now, spend $285 and buy tank-type electric water heater, roll it into your house, and save money, since the electric tankless computer heater costs average $100 more per year to operate than tank-type heater for average family of 4.
Tank type heater can be repaired by homeowner same day for $25 using off-the-shelf parts from hardware store. And that empowers the regular guy, and gets you off the junk tankless treadmill.
Gene

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

Copy following link:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html
AE115 has two elements and requires two 40 amp breakers.
A#125 has three elements and requires three 40 amp breakers, and is considered 'whole-house'
Do not install a tankless water heater because they cost more to install, and cost more to operate and maintain..
Add a comment and ask question.

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

Contact Bosch 800-283-3787, or service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Product has 10 year limited warranty against defects in workmanship, failed parts, failed heat exchanger for original homeowner with receipt. But does not cover neglect, failure to follow maintenance, installation etc. Pg 23 example manual
You generally have to pay for service call to verify installation, then send tankless into factory, or have service technician repair unit, at your expense minus cost of parts
http://waterheatertimer.org/pdf/Bosch-AE115-AE125_manual.pdf

To double check wiring:
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

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

You posted under Bosch electric tankless.
I don't know specifically how to fix your tankless from information given.
Tankless problems can be simple or complex, and include internal and external factors best gauged by experienced service technician who is there and can put hands on the appliance.
You can also glean information from the service technician how to best maintain your tankless unit, since they require yearly maintenance, such as de-liming.

Open following link for Bosch tankless troubleshooting resources, including Bosch contact information, typical manuals, error codes, tech support, parts, fuel-saver calculator, service bulletins, water heater forum, fixya links, disclaimers, and links for finding service technician.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

SOUNDS LIKE THE PUMP NEEDS REPLACING CHECK IT OUT.

I dunno ... did you turn it on?

What kind of result do you expect from this kind of question???

Hi Is this powerstar AE115 a tankless water heater? I suspect the 115 designator in the model number means it runs on 115 volts. And what is the second Tank ? Storage or an electric water heater? Both should be rapped with insulation to hold the heat. Meaning storage or electric water. Don't rap a tankless water heater. At the very least get a licensed electricen in to make sure you electrical service will support a 240 volt system. If it can , do it. Think of electricity as a river flowing. Volts are the speed the water flows. Amps are the force ( Also caled Impedeance) that push the water.( And spin your meter like a top I might add.) And resistance as how many bolders, Rocks, Old fords that are in the river bed slowing down the flow of water.Now here is where OMES LAW comes in. E= I x R. (E) volts is equel to the (I) impedeance (Amps) multiplied by the ( R) resistance. You can spin this in a clockwise direction and find the net value by dividing one from the other to get the net. ( I = R * E ) If This is true ( and it is ) if we double the voltage (current) we can realise a reduction on the Impedeance (Amps) of up to 1/2 of its current value.In other words if you were useing 100 amps @115v you will use 50 amps @240v Depending on the resistance. Sorry but I forget the voltage drop on 100 feet of copper wire but I assure you its real low.But your best bet would be to use a tankless water heater core in you boiler that you use for heat if you got one and save the electric for summer mode.