My water heater has a thermocouple with an in line overtemp disc stamped 190. The pilot will not stay on after holding the gas valve's pilot start button. So, i assumed like every other water heater i have repaired, that it must be the T/C. But i have never encountered before this, an in-line safety disc on a t/c. So i tried installing a standard universal type t/c and the pilot still will not remain on! is it possible my old t/c with the disc is creating a different mV value? the universal one screws right into the gas valve (same thread, diameter and depth). By the way the mfg of the heater is Kenmore.
By-pass the limit switch and see if the pilot light stays on. We've replaced so many of these limits, I can't count them anymore.
A few tanks have had a service bulletin issued to replace the entire gas valve / burner assembly. We've down about 8 of these so far. The parts are free at least and have solved the issue 100% of the time.
Sometimes poor venting is the issue. Check the air intake underneath the bottom of the tank if the screen wasn't installed around the base of the tank.
You're going to have to use the model number of the heater, to look up what the proper replacement part is. Replace the failed part with a proper replacement, not a generic part.
The temperature limit device is a safety device, and should not be bypassed or overridden. Doing so will likely void the warranty, and could lead to personal and/or property damage.
A while back a friend called me over for a similar problem. What I found was the over temp safety had a small reset between the electrical contacts. Pressing in on the button reset the safety and he was good to go. It is always a good idea to find the root cause in that case the burner was dirty on 1 side causing the other to be a flame thrower.