The thermal cutout, on the left side, pops up when water stops flowing through the unit. The modules are Polymer. I can press the button, have tons of hot water as long as the water is flowing through the unit. Within seconds of turning the water off in the house the thermal cutout will pop up again. If the water left in the unit is still hot the button cannot be pushed again. Once the water has cooled or starts flowing again the button can be pushed for unlimited hot water again.

SOURCE: Bosch PowerStar125 won't heat and I can't get one element to work

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.

SOURCE: BOSCH AQUASTAR 2400ES NATURAL GAS TANKLESS HOT WATER HEATER

Q. FA - Gas leakage error, gas valve circuit not closing properly
A.

1. Disconnect power supply cord and check wire connections on gas valve and the two connectors on the control board.
   
2. Flow water out of a hot water tap above the minimum activation point of .8GPM. Measure voltage at the gas valve wire connector. The connector should measure 24VDC between the left pair of wires and 24VDC between the right pair of wires when the unit is operating. If voltage is not correct, contact Bosch Water Heating for further instruction.
   
3. Gas valve may be defective, contact Bosch Water Heating for further instruction.

SOURCE: 125FX (with FXhood) turns off while in use

may be overheat stat or fan check little hose at fan area for pinhole best of luck

SOURCE: heater is ae125 is about

Hey did you ever get this fixed mine is almost the exact same thing we've been using ours for about 3 years and today nothing. Thanks for your help.

SOURCE: Bosch Pro Tankless - Shows error code E4 - every

E4 error code on Bosch tank less hot water heater, this is what it says.....

E4: Backflow temperature sensor over 309°F. The appliance will close the burner and lock.
A.
Check combustion air and backflow vent piping for blockage.
Check if secondary fan connections are disconnected.
Check venting specifications are met. Long vent lengths, venting with more than three elbows, blocked vent or combination venting may cause this failure.
Check sensor resistance (see Fig. 61, page 44 in manual).