The safety thermal cut off switch keep cutting off and am not getting any hot water in the shower

replace thermal switch, check if burner is getting enough air... clean air intake
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Gene

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

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

going off on a safety possible high temp unit needs service such a nozzle filter if oil or thermocouple if gas

Remove the wires at both ends of the thermal cut-off that is located in the little plastic sleeve below the pressure relief valve area and it's connected to spade connector on thermostat disc. Once removed, connect the female spade from wire to male spade on thermostat disc, and try to light now. If all's o.k. replace thermal cut-off as it is a safety feature to protect in case of flame up.
If not, get back to me.

Take off front cover, on the top of the cylinder(s) 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.

the thermal coupling may be bad!! if the coupling goes bad it will shut down flame. its a safety feature, if gas comes out, and no flame it shuts gas off. the coupling is in pilot light flame and works on the same principle as a thermometer to sense the heat. good luck !!

almost all tankless water heaters have this problem. I would install a small "buffer" tank. for example a small 5 or 10 gallon electric water heater will "mix" the water so that it doesn't come out of your shower cold, or scalding hot.
the instructions that came with your water heater should have a diagram of how to do this. if not let me know, I can send a copy.
the electric water heater acts as a mixing station.
the problem with most tankless heaters is that the water coming in is cold, average 60* and the water coming out is average 120*. sometimes the flame in the unit can't keep up when using large amounts of water, and you will get cold spots. when this happens, the heater will sense this, and then tells the flame it needs more BTU's, so then you will get extremely hot water.
the buffer tank will fix this by mixing the water before it gets to your shower and will give you an even water temperature.

let me know if this helps

the diagram on this page shows the safety thermal cut-off on top of the heating vessel on the right, inside the unit, remember to electrically isolate before opening.http://www.tanklesswaterheaters.com/tempra12.html

check the screen on the cold water intake on the cold side,, you might have to flush the system out, and do you have the shower head water restricter in, if not you may be using to much hot water,,, how many other hot water fixtures are running at the same time? try running just the shower,,,if this is a tank water heater the dip tube could be bad that is on the cold side it takes the cold water to the bottom of the tank and forces the hot water out,, if you ar only getting a couple of min of hot water and the dip tube is bad it is not going to the bottom but just at the top and not getting all the hot water you will need a plumber

It sounds like you have a copper canister unit. Ohm out the elements in that one canister. the directions are on the websight for bosch, or I can walk you through it. But you will need a digital multimeter to do this. If you have a blown element in that one canister, then the tco will not reset.