Our gas was shut off and when it was shut back on the tankless water heater not working. I can hear the pilot clicking but no flame.

The thermostat shuts off the main burner.The pilot light gas is controlled by the thermocouple which the side of the pilot flame has to envelop. If the flame is too small or too far away from the flame, it shuts the gas off to the pilot. The thermocouple generates a small voltage which keeps the pilot gas open. It can over time fail itself, so look at replacing it. The thermostat sounds like it's failing too.

I would question the Thermo couple. It pretty much runs the whole show. Inexpensive quick fix. It's the small tubing that is connected to the gas valve. The other end is a bulb that sits over the pilot flame.. Remove the old and match it up at the hardware store. Install new coupler ensuring bulb is directly over the pilot flame Common iten. The thermocoupler alks to the gas valve. If the pilot flame extinguishes (goes out) the bulb cools and shuts down the gas valve so the pilot gas feed doesn't permit raw gas to accumulate, avoiding the need emergency Services. . You will not get the heater to fire until the bulb is good and hot. The red push button ,when depressed will allow the pilot flame to be lit. Hold it down until the heater fires, indicating the T-coupler has reached its operating temp.Good Luck-Ned

Natural Gas: Pilot burner or orfice may need to be cleaned. Pg 11 manual.
http://www.boschhotwater.com/Portals/7/TechManuals/125BL_English.pdf
Most tank and tankless gas water heaters today suggest annual cleaning and maintenance of burner.

Open following links for Bosch troubleshooting resources:
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/125BLTechSupport/Troubleshoot/tabid/474/Default.aspx

The gas valve makes a single click sound when the pilostat shuts off. That is a safety holding coil for units with standing pilot lights and closes if it thinks the pilot went out. In electronic ignition, the same holds true except the sensor is electronic.

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 !!

The pilot light has a thermocouple that allows gas to the pilot light while the flame is burning. If flame goes out it shuts gas off to the pilot jet. It seems that your thermocouple is faulty and needs to be replaced if pilot light keeps going out as the outside air cools.

there is a small dia copper pipe which has a thermal coupling on it the end of it sits in the flame of the pilot light,
if it fails which it does after about 25-15 months,it shuts down the gas burner

Check to make sure the flame of the pilot is hitting the thermocoupling. If not then you can use compressed air to clean the pilot port and relight and check again. If the flame is hitting the thermocouple, then it may need to be replaced.

If the thermocoupler is defective the gas to everything is shut off. The coupler is the bulb that sits ib the pilot flame If it doesn't heat up it will sense no pilot and shut the gas off. They don't cost much and are easy to change. Good Luck, Ned

Thermo coupler may be defective. it controls the main gas valve by closing main if pilot is out. make sure thermo coupler bulb is positioned over and near tip of pilot flame, but not in the flame. Thermo couplers are inexpensive and easy to change out.