Water Heater Thermocouple — Diagnose & Replace

The thermocouple is the small metal rod near the pilot flame on atmospheric gas water heaters. It generates a tiny voltage (25-30 mV) from the pilot flame heat that holds the gas valve open. When the thermocouple fails, the pilot won\'t stay lit. Replacement is the most common DIY water heater repair — universal part, 45 minutes, $15-25.

How thermocouples fail

• Age: typical 5-10 year lifespan; eventually loses voltage output
• Corrosion: contact corrosion at gas valve connection reduces signal
• Position drift: tip drifted out of the pilot flame
• Oxidation: surface oxidation reduces voltage generation
• Pilot orifice clog: weak pilot can\'t heat thermocouple enough

Symptoms of failed thermocouple

• Pilot lights but goes out 5-10 seconds after releasing gas button
• Pilot stays lit while button held, dies when released
• Pilot lights but burns weakly
• Modern DSI equivalent: 2 LED flashes (AO Smith family) for thermopile low voltage

Diagnostic test (millivolt meter)

1. With pilot lit (hold gas button down), measure mV at thermocouple leads at gas valve
2. Healthy: 25-30+ mV
3. Below 20 mV: thermocouple weak; replace
4. Below 10 mV: thermocouple dead; replace immediately
5. If reading is healthy but pilot still won\'t hold, the gas valve electromagnet is the problem (not the thermocouple)

Replacement procedure

1. Turn gas valve to "Off" and let assembly cool 10 minutes
2. Remove burner access cover at the front of the water heater
3. Disconnect thermocouple at gas valve — 7/16" or 9/16" wrench on the nut at the valve body
4. Pull the burner assembly out partially to access the pilot bracket
5. Loosen thermocouple bracket screw at the pilot
6. Slide out the old thermocouple from the bracket
7. Insert new thermocouple with tip ¼" into the pilot flame envelope (top ⅓ of the blue cone)
8. Tighten bracket screw
9. Hand-tighten the nut at gas valve plus ¼-turn — do NOT overtighten
10. Reinstall burner assembly
11. Light pilot per manual; hold button 60 seconds; release; verify pilot stays lit

Critical mistakes to avoid

• Overtightening the nut at gas valve — crushes the thermocouple tip; immediate failure
• Wrong position in pilot flame — tip too far or too close to the flame
• Confusing thermocouple with thermopile — different parts (thermopile produces ~600 mV, thermocouple ~25 mV)

Cost

• Universal thermocouple: $15-25
• OEM brand-specific: $20-35
• Plumber labor if hired: $80-150

Bottom line

Thermocouple failure is the #1 reason pilot won\'t stay lit on atmospheric gas water heaters. Universal $15-25 part. 45-minute DIY. Don\'t overtighten the nut at the gas valve. Always inspect and clean the pilot orifice while you have the burner assembly out.