Gas Fireplace Won't Stay Lit

Almost certainly a weak or failed thermocouple. Here's the mechanism: when you press and hold the pilot button, you're manually holding the gas valve open. The pilot flame heats the thermocouple, which generates a small electrical signal (millivolts). That signal feeds back to the gas valve and tells it 'the pilot is lit, you can stay open after I let go.' If the thermocouple is weak, the signal is too low, the valve closes when you release the button, and the pilot dies. We test millivolt output during the diagnostic; below 14 mV on a thermocouple-driven valve, the valve won't hold. Typical fix: thermocouple replacement, $200-280 all-in.

Both generate millivolts from the pilot flame heat, but at different scales. Thermocouple = single junction, generates 18-30 mV at full pilot, drives a millivolt-only valve that holds the pilot side open. Thermopile = stack of multiple thermocouple junctions in series, generates 350-750 mV at full pilot, powers the entire valve including the main burner solenoid (no external 120V or 24V needed). Most direct-vent inserts (Heat & Glo, Heatilator, Majestic) use thermopiles. Older atmospheric-vent and some outdoor units use thermocouples. The repair pattern is similar; the part is different.

Year 4-7 typical replacement window for thermocouples. Year 6-10 for thermopiles. SoCal's seasonal use pattern (3 months active in winter, 9 months idle) accelerates oxidation: the thermocouple sits in a damp firebox all summer with no flame to burn off moisture, and the millivolt output drops. We see the failure most often on the first October or November call of the cooling season, when homeowners try to light the fireplace for the first time and discover the pilot won't hold. Coastal addresses (Newport Beach, Manhattan Beach, Malibu, Pacific Palisades) compress the timeline by 1-3 years from salt-air corrosion.

On a properly installed sealed-combustion outdoor unit, no. The combustion chamber is closed and the pilot is protected. On older outdoor fireplaces with open or partially open fireboxes (some Empire, Real Fyre, and Peterson outdoor designs), strong wind can disturb the pilot flame and cause millivolt drop, which mimics a failed thermocouple. Santa Ana wind events in October-November are the most common trigger we see. Diagnosis: if the pilot lights cleanly indoors-style but extinguishes during gusts, the problem is wind exposure rather than the thermocouple. We can install a wind-resistant pilot hood or recommend a sealed-combustion retrofit.

Thermocouple replacement: $200-280 all-in including the part ($30-60), $120 outdoor diagnostic (waived with repair), and 25-40 minutes labor. Thermopile replacement: $250-340 all-in including the part ($80-140) plus diagnostic and labor. We test millivolt output before and after the swap to confirm the fix. Best practice: replace thermocouple and pilot generator (thermopile) together when both are within 1-2 years of expected failure window, because age-matched components fail near each other and a callback in 6 months for the second one costs more than doing both at once.

Yes for the major brands. Heat & Glo, Heatilator, Majestic, Valor, Mendota, Lennox Hearth thermocouples and common thermopiles are standard van stock. Heat & Glo and Heatilator share many parts (same parent: Hearth & Home Technologies), so cross-compatibility helps. Specialty parts (Empire outdoor, Real Fyre rare models, Kingsman variants) sometimes require a 3-5 day order. We confirm part availability during the diagnostic and don't quote a part we can't source quickly.

Yes. Salt-laden marine air pits exposed metal at the pilot assembly, accelerates thermocouple junction oxidation, and corrodes the millivolt connection terminals at the gas valve. We see thermocouple failures at year 3-5 in Manhattan Beach, Marina del Rey, Newport Beach, and Malibu vs year 6-9 inland. Stocking marine-grade replacement thermocouples and the corrosion-resistant terminal hardware on coastal-route vans is part of how we keep one-visit success rates high in those neighborhoods.