Is a bad thermocouple why my water heater pilot keeps going out?

If you smell gas, do not relight

Do not try to light the pilot, use switches or use a phone inside. Leave on foot and call the gas utility and 911 from a safe distance^[6]. Everything below assumes there is no gas smell.

What a thermocouple does

On a standing-pilot gas water heater, a small pilot flame burns all the time. The tip of the thermocouple sits in that flame. Heat makes it produce a tiny electrical current, too small to detect without a meter, which flows to the gas control valve and holds its safety valve open. As long as the pilot heats the thermocouple, gas can flow. If the flame goes out, the current stops and the safety valve shuts off the gas^[3].

A.O. Smith's service handbook gives 20 to 30 millivolts as the normal output of a working thermocouple. It also warns that after the pilot goes out, it can take up to 180 seconds for the thermocouple to cool enough to close the safety valve^[3]. That is one reason lighting labels tell you to wait before trying again.

Thermopile versions. Many newer heaters use a thermopile instead: a bundle of thermocouples that makes more power. On A.O. Smith heaters with a Honeywell control, the thermopile makes 750 to 900 millivolts with nothing connected and must produce at least 350 millivolts when connected to the control. That power also runs the status light^[4].

Signs it has failed

Symptom	Likely cause	Who handles it
Pilot lights while you hold the knob, goes out when you let go	Worn thermocouple, or tip not in the flame[3]	Licensed plumber
Pilot goes out hours or days later	Weak thermocouple, drafts, dirty pilot	Licensed plumber
Status light shows low thermopile voltage	Weak thermopile or loose wire[5]	Licensed plumber
Pilot will not stay lit with a new thermocouple	Gas control safety magnet[5]	Licensed plumber
Yellow, weak or wavering pilot flame	Dirty pilot tube, not the thermocouple	Licensed plumber
Will not relight at all, lockout code	Flammable vapor sensor or other safety lockout	Licensed plumber

Rheem's troubleshooting chart for a pilot that will not stay lit lists a disconnected thermopile, a defective thermopile, and a defective safety magnet in the gas control. It says the thermopile should be replaced by a qualified service technician^[5].

What a homeowner can safely do

• Follow the lighting label once or twice. The steps printed on the heater are written for owners of standing-pilot models. Do not keep retrying.
• Use only your hand on the knob. Rheem says never to use tools, and to call a qualified technician if the knob will not turn by hand^[5].
• Look at the flame through the sight window: a steady blue flame that wraps the thermocouple tip is normal.
• Note patterns. Windy days, a dryer or bath fan running, or a recent new appliance can point to a draft problem rather than the part.
• Clear the area around the base of lint, boxes and anything that gives off fumes.

Do not remove the burner door, bend the thermocouple, or loosen its fitting at the gas valve. On sealed FVIR heaters, the combustion chamber door and its gaskets are part of the safety system, and A.O. Smith's manual says damaged combustion chamber door seals must be repaired^[8]. Leave that to a licensed plumber.

What the pro checks

A.O. Smith's handbook describes the test a technician runs: disconnect the thermocouple at the gas valve, connect a meter, light the pilot and hold the knob, and read the output after about 45 seconds. Below 20 to 30 millivolts, they reposition the tip in the flame and retest, then replace it if it still fails. If it reads fine, they move on to the gas control valve^[3]. For thermopile heaters the pass mark is 350 millivolts^[4].

That order matters for your bill. A thermocouple is cheap. A gas control valve is not. A good technician confirms which one failed before replacing anything.

What it costs

Source (date)	Item	Range
HomeGuide (Aug 2025)	Thermocouple replacement, installed	$100 to $325[1]
HomeGuide (Aug 2025)	Thermocouple, part only	about $10 to $30[1]
Modernize (Aug 2026)	Thermocouple or flame sensor repair	$150 to $350[2]
HomeGuide (Aug 2025)	Gas control valve, if the thermocouple is not the problem	$150 to $550[1]

Service call minimums of $50 to $200 may apply^[1]. These are national ranges, not quotes.

Because the part is inexpensive and the job is routine, a thermocouple repair usually makes sense even on a middle-aged tank. Rheem's guidance is to repair units under about 6 to 8 years old and to replace when a repair costs more than 50% of a new unit^[7]. That rule is more likely to bite if the diagnosis turns out to be the gas valve.

When to call a pro

• The pilot will not stay lit after one or two tries with the lighting label.
• The status light shows low thermopile voltage or a lockout.
• The pilot flame is yellow or weak, or blows out repeatedly.
• Any replacement of the thermocouple, thermopile, pilot or gas valve.