I need a safety thermalcut off switch for my Tempra 24+

There are several potential causes of low water pressure with a tankless water heater like the Tempra 24 Plus:

1. Flow rate limitations: Tankless water heaters require a minimum flow rate to operate properly, typically around 0.5-0.6 gallons per minute (GPM). If the flow rate drops below this threshold, the heater may shut off or fail to produce hot water. In your case, the initial flow rate of 3.4 GPM is well above this minimum, but the drop to 1.6 GPM suggests that there may be a flow restriction or other issue causing the lower pressure.
2. Water supply issues: Low water pressure can also be caused by problems with the water supply itself, such as a clogged or damaged pipe, a water main break, or problems with the city water system. Check with your local water utility to see if there are any known issues or maintenance activities that could be affecting water pressure in your area.
3. Water filter or softener issues: The whole-house water filter and softener that you installed could also be causing low water pressure if they are clogged or not functioning properly. Check to make sure that the filters are clean and that the softener is operating correctly.
4. Plumbing issues: There could be issues with the plumbing in your home, such as a clogged or damaged pipe, a faulty pressure regulator, or incorrect pipe sizing. Have a licensed plumber inspect your plumbing system to identify any potential issues.

If you have already checked for these potential causes and are still experiencing low water pressure with your Tempra 24 Plus, it may be helpful to contact the manufacturer or a licensed technician for further assistance.

If you have already descaled your Tempra 24 Tankless water heater and replaced the flow sensor, but the high limit switch is still tripping, there could be a few other possible causes of the issue: Malfunctioning thermostat: A malfunctioning thermostat may cause the heating modules to overheat, which can trip the high limit switch. You may need to replace the thermostat if this is the case. Insufficient water flow: The Tempra 24 Tankless water heater requires a minimum flow rate of 0.5 gallons per minute to operate properly. If the flow rate is too low, the heating modules may overheat and trip the high limit switch. Check the water flow rate to ensure it meets the minimum requirement. Electrical issues: If the wiring or electrical connections are faulty, it may cause the heating modules to overheat and trip the high limit switch. You may need to inspect the wiring and connections to ensure they are intact and properly connected. If none of the above solutions work, it may be best to contact a professional technician for further diagnosis and repair. It is unlikely that you would need to replace the entire unit if the issue is isolated to a single heating module.

First, check the circuit breaker. Also, did you upgrade to heavier wiring and a double 60 circuit breaker when you had this installed?

Check power make sure you have enough going to it as well as make sure the filter in it isn't plugged up.

have u turned water back on, also some times water heater has a one way valve fitted if put back on wrong way round it wont work

hope this is what you need.
http://www.stiebel-eltron-usa.com/techdata_tempranew.html

http://www.stiebel-eltron-usa.com/pdf/install_tempra_tempraplus.pdf

This is common problem with tank-less electric water heaters. There a a couple of things you can do. Turn the temperature down on the thermostat, this will lower the power requirement. The other thing you can do is replace the safety thermal cut-off switch. If you have two or three replace then all at the same time. Thermal safety switches are manufactured by the hundreds of thousands and go bad quit often. Contact the manufacture for free replacement parts.

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

Did you turn the breaker completely off, then back on? When a breaker trips it is in a middle position, and nust be turned off to reset it, then it can be turned back on.

On the Tempra 29 there is a "safety thermal cut-out" on the top of each of the heating elements within the box. I have had to open it up before to push the breaker. They are a small red plastic push button on the top. Carefully press the braker button without touching the electrical wiring inside. I hope that works for you.