Thermal breaker keeps tripping only 4 years old. clean water and change filter regularly.

You can find this site but you can't google the manufacturer site.

If you are not able to resolve a problem please contact us toll free at 800-582-8423 before removing the unit from the wall. Stiebel Eltron is happy to provide technical assistance. In most instances, we can resolve the problem over the phone.

Replace the limit, by tripping often they become even more likely to nuisance trip.

Type, model, brand, age of water heater, and general installation of water heater are unknown.

1) Electric tank-type water heater: replace thermostats and test elements.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

2) Gas tank-type: replace gas control valve, clean air intake screen/ location varies by brand:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

3) Tankless: call service technician, or replace tankless with tank-type and save money long-term.

4) Add comment and describe water heater using label located on side of unit, and include what you are doing to 'reset' the thermal cut-out.

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

Water heater has upper and lower elements. Only one t-stat. Check to see where tstat is set to. Turn down to lowest level. If breaker still resets then replace tstat. It is letting the water get to hot no matter what setting it's on. So water is getting so hot that it trips breaker.

All resets and breakers trip with high heat.
Do you mean the tankless unit is tripping the 40 amp circuit breaker?
Or the thermal cut-out, located on unit, is tripping?

Is this a new installation?
Or was the tankless working before and stop working?

Copy following link to check wiring.
AE 9.5 requires 40 amp breaker and 6 gauge wire.
http://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.html

If the unit is tripping the thermal cut, -out, then product manual says to call Bosch or service technician.
Call Bosch 866-330-2729
http://waterheatertimer.org/pdf/Bosch-Powerstar-AE3.4-7.2-9.5.pdf
Thermal cut-out can trip if water flow is too low.
Try changing power selector screw from Hi to Lo so only 1 element is ON. See diagram page 6 of manual.
Adjust the 'outlet service valve:' The manual recommends that a ball valve be installed between hot outlet and faucet. Adjust the valve so more water flows to faucet.
Remove aerator, clean aerator, don't use low flow restrictions or low flow shower heads, replace leaky faucet cartridge on single-handle faucet.

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

Copy following link for troubleshoot walk-thru for electric water heater tripping breaker:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker

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

Hello; I can't clain any formal training or education in electrical, But, I am a home owner and Mechanic so I'll give this a try. Thermal switches are in the norm based on the bi- metal concept/ Automotive flashers are in essence thermal switches. Of course there are electronic types that contain points and a coil, we can forget those since we aren't doing a car or truck, let alone turn signal. OK, moving on. As you know,excessive continuous heat generated by a excess load will destroy the bi-metal strip which serves as a bridge in the circuit. A defective flasher will increase in speed as it gets older and used heavily. Anyway, to the point. I've experience thermal switches that cease to function if trip too often or were tripped by an excessive load. Although resettable initially, it's thermal tolerance gets shorter and shorter until eventually you get irritated enough to ask someone on Fix-ya for some input. No problem. I reccommend replacing the breaker with or of same amperes. You may want to verify the load by checking the appliance data plate just in case it's the straw the breaks the circuit I hope I helped you and I apologize for the analogy used. Thanks for using fix-ya. Good Luck _Ned_t

Take off front cover, on the top of the two cylinders in there (the heating system) there will be a small red button. That is the Safety Thermal Cut out, depress that red button. It is similar to a circuit breaker. If the red LED comes on when you turn on hot water at a sink then that was the problem. I have on that trips periodically. Its annoying but not enough to replace the part yet. Good luck.

Check the voltage at the water heater. Sounds like the element is burnt out and is tripping the breaker.

If it is more then 5 years old I would check.install a new heating element.