If the thermocouple is tight in the thermostat, then the next most likely problem is that the pilot burner is dirty and the flame is not contacting the thermocouple or is not getting it hot enough.: Need info on a water heater that cannot keep the pilot light lit. We have replaced the thermocouple already as well as the regulator. Thinking of changing out the thermostat–only thing left we can think of. Any ideas. This equipment came with the house and were assuming it was new when house was built–in 1991. We have propane, btw. Thanks in advance!