Gas Fireplace Pilot Light Won't Stay Lit — 5 Causes & Fixes

A pilot light that won't stay lit is frustrating — and surprisingly common. The good news: it's almost always diagnosable and fixable in a single visit. Here are the five most likely causes.

1. Faulty Thermocouple

The thermocouple is a small safety sensor that detects whether the pilot flame is lit. When it's working correctly, it sends a signal to keep the gas valve open. When it fails or gets coated in residue, it tells the valve to close — even with the pilot on. This is the most common cause of a pilot that won't stay lit.

Fix: Thermocouple replacement. Usually a 30–45 minute job for a technician.

2. Weak Thermopile

The thermopile generates the small electrical charge needed to power your fireplace's control valve and receiver. A thermopile that's producing low millivolts can cause intermittent shutoffs, especially in colder weather. You can test this with a multimeter — a healthy thermopile produces 300–750 mV.

Fix: Thermopile replacement or cleaning.

3. Dirty Pilot Assembly

Dust, spider webs, and debris can clog the pilot orifice over time, resulting in a weak or yellow flame instead of a strong blue one. A weak flame doesn't generate enough heat for the thermocouple to register.

Fix: Professional cleaning of the pilot assembly and orifice.

4. Gas Supply Issue

Low gas pressure or a partially closed shutoff valve can result in an insufficient flame. Check that the manual shutoff near the fireplace is fully open. If you recently had gas work done in your home, a pressure issue is worth investigating.

Fix: Check the shutoff valve; if pressure is suspect, call your gas utility or a licensed technician.

5. Faulty Gas Valve

If the thermocouple and thermopile test fine, the issue may be the gas valve itself. Gas valve failures are less common but do occur, especially in older units.

Fix: Gas valve replacement. This requires a licensed technician.

When to Call a Professional

If your pilot light won't stay lit after two or three attempts, stop trying and call a technician. Repeated relighting without resolving the root cause can allow unburned gas to accumulate. Learn more about our gas fireplace repair services or schedule a visit today.