Water Heater Pilot Light Won't Stay Lit? A Field Tech's Diagnostic Flow

Before touching anything, pay attention to exactly what happens when you try to light the pilot. The failure pattern tells you what to check first:

• Pilot won't light at all: No gas reaching the pilot — check gas supply and shutoff valve before anything else.
• Pilot lights while holding button, goes out immediately when released: Classic thermocouple failure. The valve isn't receiving the hold signal. Start at Step 2.
• Pilot lights, holds for a minute or two, then goes out: Thermocouple may be marginal (low millivolt output) or a draft is extinguishing the flame. Check thermocouple output and inspect the flue.
• Pilot lights and holds, but main burner won't fire: The thermocouple isn't the issue — the gas valve, thermostat, or main burner orifice is likely the problem.

The thermocouple is a thin copper tube — typically 18–36 inches long — with a probe tip positioned directly in the pilot flame and a connector threaded into the gas valve. To test it:

• Set your multimeter to millivolts (mV) DC
• Light the pilot and hold the button for 60 seconds to fully heat the tip
• Clip the multimeter leads to the thermocouple connector (on the valve) with the thermocouple still connected
• A good thermocouple reads 25–35 mV when hot. Below 15 mV means it's failing.
• Under 20 mV: replace the thermocouple, even if it's technically "in spec" — it'll fail soon

To replace: shut off the gas at the manual valve, unscrew the thermocouple from the gas valve (usually a 3/8” nut), slide the probe tip out of the pilot bracket, and reverse with the new unit. Universal thermocouples ($10–20 at any hardware store) fit most residential tanks.

If the thermocouple tests good but the pilot still won't hold — or the flame looks small and yellowish instead of a strong blue cone — the pilot orifice is partially blocked. Spiders particularly like to nest in gas lines and pilot assemblies during warm months. Even a partial web across the orifice is enough to cut flame output by 30–50%.

To clean:

• Shut off gas at the manual valve
• Disconnect the pilot tube from the gas valve
• Remove the pilot assembly from the combustion chamber
• Use a can of compressed air to blow through the orifice — do not use a wire or drill bit, which will enlarge the orifice and change the flame characteristics
• Inspect the orifice opening visually with a flashlight — it should be clean and round
• Reassemble, restore gas, relight, and observe flame shape

A pilot that holds normally but goes out randomly — particularly when exhaust fans are running or doors are opened — is almost always a draft or negative pressure problem. The combustion air intake and flue on a standard atmospheric water heater are not sealed; they rely on natural draft.

Check in this order:

• Flue pipe condition: The draft hood above the tank and the flue pipe to the exterior should be intact with no holes, disconnected sections, or bird nests at the termination cap
• Combustion air supply: The mechanical room needs adequate combustion air — typically 1 sq. inch free area per 1,000 BTU of appliance input. A sealed or undersized room starves the burner of air and creates negative pressure
• Competing exhausts: A nearby bathroom exhaust fan, range hood, or HVAC air handler can create negative pressure in the space. Light the pilot and then turn on all exhaust fans — if the pilot goes out, you have a depressurization problem
• Flue check: Hold a lit match near the draft hood with the burner running — smoke should be drawn up into the hood. If smoke blows outward, the flue is not drafting and needs inspection

Low gas pressure is an underdiagnosed cause of pilot problems — the flame looks lit but is too weak to adequately heat the thermocouple. If Steps 1–4 haven't resolved the issue, verify gas supply before replacing the valve.

• Manual shutoff valve: Confirm it's fully open — parallel to the pipe. A partially closed valve is easy to miss and reduces pressure significantly
• Other gas appliances: Light the range or check the furnace. If they're also underperforming, the issue is upstream — contact the gas utility for a pressure check at the meter
• Proper pressure reading: Residential natural gas supply pressure at the appliance should be 3.5–7” water column (WC). Propane systems typically require 10–11” WC at the appliance. Low pressure requires a regulator inspection or utility call — not a water heater repair

If you've replaced the thermocouple, confirmed the pilot orifice is clear, ruled out draft and gas supply issues, and the pilot still won't hold — the gas valve is the next suspect. On most residential water heaters, the entire valve assembly (including the thermostat dial) is one replaceable unit.

Before replacing:

• Confirm the new thermocouple is generating adequate millivoltage (25+ mV, measured at the valve connection)
• Inspect the thermocouple connection at the valve — the nut should be snug but not overtightened (finger tight plus 1/4 turn). A loose connection produces the same symptom as a failed thermocouple
• On Honeywell WV8840/WV8860 valves (common on Rheem, Bradford White, State): check the pilot valve button for physical damage — it should depress smoothly and spring back fully. A sticky or damaged button holds the pilot valve partially open and prevents normal operation

Gas valve replacement requires shutting off the gas supply, draining the tank or isolating the valve, and disconnecting all gas, thermocouple, pilot tube, and thermostat connections. Match the replacement exactly by model number from the unit's data plate — valve thread sizing and BTU rating are both critical.