How to Test a Water Heater Thermocouple
Updated February 25, 2026
Test your gas water heater thermocouple with a multimeter to determine if it is the reason your pilot light keeps going out -- a 10-minute diagnostic.
Overview
The thermocouple is a safety device that sits in the pilot flame. When heated, it generates a tiny voltage (20-30 millivolts) that holds the gas valve open. When the pilot goes out, the thermocouple cools, voltage drops to zero, and the gas valve shuts off -- preventing unburned gas from filling the room. A failed thermocouple produces insufficient voltage even when heated, so the gas valve closes and the pilot goes out every time you release the control knob. Testing it with a multimeter takes 10 minutes and tells you definitively whether to replace it or look elsewhere.
What You'll Need
Safety First
- If you smell gas at any point, stop. Do not use open flames or electrical devices. Leave the area and call your gas company emergency line.
- The thermocouple tip is in a flame. Use pliers or let it cool before handling after testing.
- Work in a well-ventilated area. The pilot produces a small amount of combustion byproducts.
Step-by-Step Instructions
- Access the Thermocouple
Remove the outer access panel at the bottom of the water heater. Locate the thermocouple -- a thin copper tube running from the pilot assembly to the gas control valve. One end sits in the pilot flame. The other end screws into the gas valve with a small nut (typically 7/16-inch). The thermocouple is usually next to the pilot tube and the igniter.
Tip: Take a photo before disconnecting anything. The pilot assembly has three connections at the gas valve: the pilot tube, the thermocouple, and sometimes a thermopile (thicker version on electronic ignition models). You need to identify which one is the thermocouple. - Visual Inspection First
Before testing with a meter, check for obvious problems. Is the thermocouple tip fully in the pilot flame? It should be engulfed by the blue cone, not beside it or above it. Is the copper tube kinked, corroded, or cracked? Is the connection at the gas valve loose? A thermocouple that has been knocked out of position or has a loose connection mimics a failure but does not need replacement -- just repositioning and tightening.
Tip: Green corrosion on the copper tube near the gas valve connection is common and usually not a problem. Heavy corrosion at the tip (where it sits in the flame) means the thermocouple is degrading and should be replaced even if it still tests OK. - Test with a Multimeter (Millivolt Setting)
Set your multimeter to DC millivolts (mV). Light the pilot and hold the gas control knob down. Wait 60 seconds for the thermocouple to heat fully. Disconnect the thermocouple from the gas valve (7/16-inch wrench, keep the pilot lit by holding the knob). Touch the multimeter probes: one to the thermocouple tip connector, one to the outer casing of the connector. A good thermocouple reads 25-35 millivolts. Below 20 mV and the gas valve cannot stay open. Below 10 mV is a definite failure.
Tip: If you do not have a multimeter, there is a simpler (less precise) test: light the pilot, hold the knob for 60 seconds, release. If the pilot goes out within 10 seconds, the thermocouple is almost certainly bad. If it stays lit for 30+ seconds then goes out, the thermocouple may be marginal or the gas valve is the issue. - Replace If Failed
Thermocouple reads below 20 mV? Replace it. Turn off the gas. Unscrew the old thermocouple from the gas valve (7/16-inch wrench). Pull the tip out of the pilot assembly bracket. Push the new thermocouple tip into the bracket -- it should sit in the same position, fully in the pilot flame path. Thread the connector into the gas valve. Finger-tight plus 1/4 turn with the wrench. Do not overtighten -- you will crush the soft copper fitting.
Tip: Universal thermocouples ($5-$8) fit most residential gas water heaters. They come in 18, 24, and 30-inch lengths. Buy the same length as the old one. Too long works (coil the excess gently). Too short does not reach.
Pro Tips
- A multimeter that reads millivolts costs $15-$25 and is useful for dozens of household diagnostics beyond thermocouples -- testing batteries, outlets, fuses, and continuity. Worth owning.
- Thermocouples are wear items with a 5-7 year lifespan. If yours is original and the water heater is 5+ years old, just replace it rather than testing. The $5-$12 part is cheaper than the time spent diagnosing.
- Some newer water heaters use a thermopile instead of a thermocouple. A thermopile looks similar but generates higher voltage (300-750 mV) to power electronic gas valves. Test the same way but expect higher readings. Below 300 mV on a thermopile means replacement.
- Keep a spare thermocouple in your toolbox. When the pilot goes out at 10 PM in January, a $5 part and 10 minutes of work gets your hot water back immediately.
- If the new thermocouple also fails to keep the pilot lit, the gas valve itself is likely defective. Gas valve replacement is $150-$300 and usually requires a professional.
When to Call a Pro
Call a plumber or HVAC tech if the thermocouple tests good but the pilot still goes out (gas valve issue), if you smell gas during testing, if the gas valve connections are corroded and will not unscrew without risking damage, or if your water heater uses a sealed combustion chamber that you cannot access for testing.
Frequently Asked Questions
When should I call a plumber to test a water heater thermocouple?
Call a plumber or HVAC tech if the thermocouple tests good but the pilot still goes out (gas valve issue), if you smell gas during testing, if the gas valve connections are corroded and will not unscrew without risking damage, or if your water heater uses a sealed combustion chamber that you cannot access for testing.
What are some expert tips to test a water heater thermocouple?
A multimeter that reads millivolts costs $15-$25 and is useful for dozens of household diagnostics beyond thermocouples -- testing batteries, outlets, fuses, and continuity. Worth owning. Thermocouples are wear items with a 5-7 year lifespan. If yours is original and the water heater is 5+ years old, just replace it rather than testing. The $5-$12 part is cheaper than the time spent diagnosing. Some newer water heaters use a thermopile instead of a thermocouple. A thermopile looks similar but generates higher voltage (300-750 mV) to power electronic gas valves. Test the same way but expect higher readings. Below 300 mV on a thermopile means replacement. Keep a spare thermocouple in your toolbox. When the pilot goes out at 10 PM in January, a $5 part and 10 minutes of work gets your hot water back immediately. If the new thermocouple also fails to keep the pilot lit, the gas valve itself is likely defective. Gas valve replacement is $150-$300 and usually requires a professional.
What tools do I need to test a water heater thermocouple?
You will need: Multimeter with millivolt (mV) DC setting ($15-$25), 7/16-inch wrench (for thermocouple connection), Long lighter or fireplace matches (to light the pilot), Replacement thermocouple ($5-$12 universal, if test fails).
When should I call a professional instead of doing this myself?
Call a plumber or HVAC tech if the thermocouple tests good but the pilot still goes out (gas valve issue), if you smell gas during testing, if the gas valve connections are corroded and will not unscrew without risking damage, or if your water heater uses a sealed combustion chamber that you cannot access for testing.
How difficult is this project?
This project is rated easy. Most homeowners with basic tools can complete it.
How long does this take?
Plan for approximately 10 - 15 minutes. First-timers may need extra time.
How much will this cost?
The estimated cost is $0 (multimeter needed); $5 - $12 for replacement. Costs vary by location and materials chosen.