How to Test a Water Heater Thermocouple
To test a thermocouple, disconnect it from the gas control valve, heat the tip in a flame, and read its output with a multimeter set to DC millivolts. Honeywell puts normal open-circuit output for its standard thermocouples at 26 to 32 mV, with 18 mV as the lowest acceptable reading, so a reading well under that means the part is done.
Before you test anything, try the free check: if the pilot lights while you hold the knob but goes out when you let go, the thermocouple is the prime suspect. The test tells you whether to replace it or look elsewhere.
Signs of a bad thermocouple
You probably have a weak thermocouple if:
- The pilot lights but goes out as soon as you release the knob.
- You have to hold the knob longer and longer to get it to stay lit.
- The pilot stays lit for a while, then drops out for no clear reason.
- The tip is coated in soot or crusty buildup, or the copper lead is kinked near either end.
A.O. Smith says a pilot can also go out from drafts, too little air around the heater, gas pressure changes or a kinked or clogged gas tube. That's why you test before you buy.
Tools you need
- A digital multimeter with a DC millivolt (mV) range. Honeywell's own procedure calls for a millivoltmeter, and any decent multimeter that reads low DC millivolts will do.
- Alligator clip leads, so you're not holding probes near a flame.
- An open-end wrench that fits the thermocouple nut at the gas control.
- A long lighter or small torch.
- Fine emery cloth or a fine abrasive pad, for cleaning.
- Soapy water in a spray bottle, for the leak check afterward.
Step by step: the open-circuit test
This is the test most homeowners can do. It measures what the thermocouple can produce with nothing connected.
Shut it down
Turn the gas control knob to OFF and close the gas supply valve to the heater. Remove the outer and inner burner access covers.
Disconnect the lead at the gas control
Unthread the thermocouple nut from the gas control valve. On many heaters you'll pull the burner assembly out to free the thermocouple tip from its bracket. Angi's thermocouple guide walks through removing the burner assembly.
Clip on the meter
Set the multimeter to DC millivolts. Clip one lead to the copper sheath of the thermocouple and the other to the center contact on the end that threads into the gas control.
Heat the tip
Hold the lighter flame on the tip, the last 3/8 to 1/2 inch, which matches the pilot flame coverage Honeywell specifies. Keep heating until the reading stops climbing.
Read it
Compare the steady reading with Honeywell's figures: normal is 26 to 32 mV, and below 18 mV is not acceptable.
Reading the results
| Reading (open circuit) | What it means |
|---|---|
| 26 to 32 mV | Normal for a Honeywell standard thermocouple |
| 18 to 25 mV | Below normal but above Honeywell's minimum; clean it and check the pilot flame |
| Under 18 mV | Below the lowest acceptable output; replace it |
| Zero | Open circuit; the thermocouple is dead |
These figures come from Honeywell's sheet. Other brands publish their own specs, so check the packaging if your thermocouple is a different make.
If the open-circuit reading is fine but the pilot still won't stay lit, the problem is somewhere else. The connection at the gas control may have been loose, the pilot flame may not be hitting the tip, or the magnet in the gas control may be failing. A.O. Smith says a failing control valve usually needs a service visit and likely a new valve.
A closed-circuit test, with the thermocouple connected and loaded by the gas valve, shows whether the valve's magnet is drawing it down. It needs an inline adapter and the pilot running, so leave that one to a technician.
How to clean a thermocouple
If the tip is sooty or crusted, cleaning it can bring a marginal reading back up.
With the gas off and the tip cool, rub the tip lightly with fine emery cloth until you see clean metal.
Wipe off the dust. Don't use oil or cleaners on it.
Check the pilot burner too. A.O. Smith's manual lists soot and carbon buildup on the main and pilot burners and says the burner may be cleaned with soap and hot water.
Reinstall and retest.
Soot on the tip usually means the pilot flame is dirty, so a clean tip that sooted up again is telling you to fix the flame.
Putting it back together
Thread the thermocouple nut into the gas control finger tight, then give it 1/4 turn more with a wrench, per Honeywell. Overtightening can damage the contact. Seat the tip back in its bracket so the pilot flame wraps the top 3/8 to 1/2 inch.
Reinstall the burner and covers, open the gas, and relight using how to light a water heater pilot light. Spray soapy water on any gas fitting you loosened; bubbles mean a leak. Honeywell says to leak-test any time work is done on the system.
Mistakes to avoid
- Testing with the meter on AC or on a volts range. The reading is in DC millivolts.
- Heating only the side of the tip, then condemning a good part.
- Kinking the lead. Honeywell warns not to bend it within 1/2 inch of either end.
- Cranking the nut. Snug plus a quarter turn is the spec.
- Skipping the leak test.
When it's a pro job
Call a plumber or gas tech if the readings are fine and the pilot still drops out, if you'd have to pull a sealed-combustion burner assembly with gaskets, or if you can't get the gas fittings to pass the bubble test. HomeGuide puts a pro thermocouple replacement at $100 to $325.
FAQ
How do I know if my thermocouple is bad?
The usual sign is a pilot that lights while you hold the knob and goes out when you let go. Confirm it with a millivolt test: under 18 mV on a Honeywell standard thermocouple is a failed part.
Can I test a thermocouple without a multimeter?
Not reliably. You can watch whether the pilot stays lit after a careful relight, but that doesn't separate a weak thermocouple from a weak flame or a bad gas valve. A basic multimeter is cheaper than guessing on parts.
How do you clean a water heater thermocouple?
Shut off the gas, let it cool, and rub the tip lightly with fine emery cloth until it shows clean metal. Then fix whatever made it sooty, usually a dirty pilot burner.
What does a bad thermocouple look like?
Often it looks fine. When you can see trouble, it's heavy soot, crusty buildup or a cracked or kinked lead. The meter is the real test.
Sources
- Resideo (Honeywell)Thermocouple Installation Instructions (62-3031) (opens in a new tab)
- A. O. SmithHow to Relight a Water Heater's Pilot Light (opens in a new tab)
- AngiHow to Replace a Thermocouple on a Water Heater (opens in a new tab)
- A. O. SmithProMax Atmospheric Vent Gas Water Heater Instruction Manual (opens in a new tab)
- HomeGuideWater Heater Repair Cost (opens in a new tab)