Water Heater Thermocouple Problems
The Short Answer: A standing-pilot thermocouple that will not hold the magnet is a tip out of the flame, a crushed nut, an open generator, or a dead valve solenoid. It should make ~20–30 mV open-circuit with a sharp blue flame wrapping the tip. A millivolt valve wants a thermopile in the hundreds of millivolts. Electronic FVIR tanks do not take a hardware-store thermocouple at all. Do not jumper it. Hire a licensed plumber.
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Search Intent & The Real Problem
You searched “thermocouple problems” because the internet sold you a $12 copper probe. Sometimes that is the part. Sometimes you bought a 30 mV thermocouple for a powerpile valve, sandpapered a brand-new tip, or ignored a lint-packed FVIR screen that was blowing the flame off the generator. This page is the millivolt sensor itself — identify it, prove it, stop replacing it. Year-round dropout with no part in mind: will not stay lit. Already swapped one: still dies after a new thermocouple. Honeywell-stamped control: Honeywell thermocouple. Pilot holds, main never opens: pilot lit, burners dead.
What other guides get wrong
They treat every gas valve as a 30 mV thermocouple. Millivolt / ICON / powerpile valves need a thermopile. Electronic Rheem/Ruud/Richmond FVIR valves with a status LED are not a field thermocouple swap — Richmond gas valve. They tell you to crack the union for air on a heater that ran yesterday. They sandpaper a new tip to a needle. And they never mention the magnet: a healthy thermocouple cannot hold a dead solenoid.
Step-by-Step Troubleshooting
Step 1: Prove you even have a thermocouple
| What is on the valve | Generator it wants | Hardware-store thermocouple? |
|---|---|---|
| Knob OFF / PILOT / ON, thin single copper probe, no LED | Two-wire thermocouple, ~20–30 mV open-circuit | Maybe — match length and the old tip |
| Thicker two-lead generator, millivolt / powerpile stamp | Thermopile, hundreds of mV (often ~500–750 mV open) | No. That 18” 30 mV part will not run the main |
| Status LED, “do not light with a match,” FVIR door | Electronic flame sense — not a field thermocouple | No. Count the sticker |
| Bradford White ICON atmospheric | Thermopile | No — weak pilot signal |
| No flame at all, dark port | Supply, piezo, lockout | Not a sensor — burner not lighting |
- What to look for: The part you would pull. Thin copper vs fat two-lead pile. Sticker: millivolt vs electronic. Rating plate for NG vs LP. Photo of the valve face before anyone orders.
- Required tools: Flashlight, phone camera. No wrenches yet.
If the control is electronic FVIR, stop hunting a thermocouple. That search is how you waste a Saturday and still have a cold tank. Do not match-light a chamber the jacket forbids.
Step 2: Split “dies on release” from “never lights” from “main never opens”
| What you see | What it is | Next |
|---|---|---|
| Pilot lights, dies the instant the knob leaves PILOT | Thermocouple path, crushed nut, or magnet | This page |
| No spark, no pilot, dark port | Supply, piezo, lockout | Piezo, no gas |
| Pilot holds, main never whooshes | Thermopile under load, or the operator | Main never opens |
| Main whooshes, then drops; pilot stays | Thermopile sag / operator | Burner won’t stay lit |
| Water sputtering on the tip | Leak tracking into the door | Dripping on the pilot |
| Dies on windy nights only | Draft / chimney / dryer | Cold weather |
| Whoosh / bang, then dark | Delayed ignition | Shut down. Not a second try. |
- What to look for: One lighting sequence per the jacket after five minutes off, area clear, no odor. Pilot should wrap the top 3/8–1/2” of the tip with a sharp blue flame.
- Required tools: The lighting card on that valve, not a forum ritual.
A lazy yellow cap will not make millivolts. That is orifice, lint, or gas quality — not a reason to install a second cheap sensor. Rheem-family hold failures: Rheem pilot.
Step 3: Tip, nut, and insulation — visual only
- What to look for: Tip in the flame, not above it, not bent against the hood. After a DIY swap the bracket is often kicked up. Thermocouple nut at the valve: snug plus about a quarter turn. Gorilla-tight cracks the insulator and shorts millivolts to the chassis. Lead not pinched under the burner door. Insulation not melted through to ground.
- Required tools: Eyes at the view port. Gas cock off if you are going near the door. Do not pull an FVIR combustion door you cannot reseal.
The thermocouple nut is a millivolt connection. It is still not an excuse to crack the pilot tube or the supply union. Those are gas fittings. A brand-new thermocouple out of the envelope reads like a dead one if the tip is sitting in air.
Step 4: Millivolts — prove the generator before another part
| Reading (qualified meter, flame wrapping the tip) | Meaning |
|---|---|
| Thermocouple < ~15 mV open-circuit | Bad part, bad connection, or tip still out of the flame |
| Thermocouple ~20–30 mV open, ~12–18 mV closed, still drops | Valve magnet / ECO — stop replacing sensors |
| Thermopile < ~400 mV open, or sags under ~180 mV with the main called | Weak pile, lazy pilot, or a failing operator |
| Correct millivolts, will not hold | The gas valve. Period. |
| 30 mV on a millivolt valve | You installed the wrong generator |
- What to look for: Numbers with flame on the tip. Dropout on many magnets is only a few millivolts. Confirm that valve’s spec, not a forum. A 30 mV reading on a millivolt valve means the Amazon title lied.
- Required tools: Millivolt meter if you are qualified. If you are not, this is the hire point — not a third sensor.
Manifold pressure is licensed: typically ~3.5–7 in. w.c. natural gas / ~10–11 in. w.c. LP at the appliance — confirm the rating plate. A thermocouple does not measure gas pressure. Robertshaw Unitrol cousins: Unitrol.
Step 5: ECO, draft, FVIR — then the valve, not a fourth sensor
- What to look for: Reset / ECO button on some combination valves (tank already too hot, or a failed limit). Pilot that dies when a bath fan starts. Lint packed in the FVIR arrestor under a post-2003 tank. Chamber / vapor sensor lockout on electronic controls — chamber sensor. Water in the burner tray or a rust ring: the tank is dead; shut gas. Correct millivolts, clean draft, ECO reset, and it still dies when you let go: internal magnet. Match gas type, millivolt vs thermocouple vs electronic, FVIR.
- Required tools: Eyes. Vacuum around the jacket from outside. On a licensed truck: millivolt meter, manometer, leak-detection solution. Not a “universal” valve from a bin.
Odor during a relight: leave. Need it off while you wait? Apartment shutoff.
When DIY Ends: Calling a Licensed Pro
You may photograph the valve, confirm thermocouple vs thermopile vs LED, look at tip wrap through the port, and follow the jacket once. Stop at gas odor, a second failed sensor, millivolt numbers you cannot take, an FVIR door, and any valve replacement. You may not jumper the generator, hold the knob in, or loosen unions. A licensed plumber will measure millivolts, prove the magnet, set manifold pressure, and leak-test a matched control. Two $12 sensors and a contaminated $200 valve is the expensive order.
Frequently Asked Questions
Q: What are common water heater thermocouple problems? A: Tip out of the flame, a crushed insulator nut, an open generator that cannot make ~20–30 mV, the wrong part on a thermopile valve, water on the tip, or a dead magnet blamed on the sensor. Electronic FVIR valves are not thermocouple jobs.
Q: How do I know if the thermocouple or the gas valve is bad? A: Measure millivolts with a good blue flame wrapping the tip. ~20–30 mV open and it still drops is the magnet. Weak millivolts with the tip in the flame is the generator or the connection. Do not jumper it to “test.”
Q: Can I replace a water heater thermocouple myself? A: On an old non-sealed standing-pilot valve, matching the old generator and snugging the nut a quarter turn is a common parts job — still licensed fuel-gas work in many cities if you cannot leak-test. Pulling an FVIR burner to reach the tip is a combustion-seal job. Hire it if the first honest sensor fails.