Pictured above is a typical gas water heater's TCO (Thermal Cut-Off), where it resides on the heater's combustion chamber wall. (The TCO's two wire connections have been removed in the photo.) The TCO is there as a safety feature -- in the event of an over-heat condition in the water heater's combustion chamber, the TCO will go open, disabling the burner and the pilot light.
A defective TCO that goes open for no good reason will cause a very deceptive trouble symptom to arise; i.e. the pilot light will be disabled. You'll still be able to relight the pilot light, but the pilot light won't stay lit -- exactly the same symptom you'd expect from a bad thermocouple.
A continuity test with an ohmmeter will not necessarily reveal the fault in the TCO. The TCO may check out ok with an ohmmeter applied to it, but fail intermittently while in service nonetheless. To be absolutely, positively certain whether or not you have a TCO fault, bypass the TCO altogether with a splice connector, like so.
If the water heater works reliably with the TCO bypassed, you'll know that the TCO is at fault, not the thermocouple.
The TCO in the above photographs gave me exactly the trouble I've just outlined. One of its spade terminals is a little bit wiggly; that may be the cause of its intermittent, deceptive behaviour. Here's a close-up view of the thing.
The spade terminal at the unit's upper right is the wiggly one. I may be able to repair that with a hammer and punch. If not, I'll have to obtain a replacement.
In any event, I've saved myself a costly service call, and the water heater is back in business for the time being with its TCO bypassed. I'll take my chances with the safety feature's absence.
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If you find the boiler pressure loss is only when the heating is on, that’s a sign of a small leak. The excess pressure caused by increased heat puts extra strain on the component leaking.
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