Pilot won't stay lit?
Pilot Light & Thermocouple Repair in Denver, CO
It lights, you hold the button, you let go, and it dies. That specific sequence points at one part more than any other.
- Thermocouple and thermopile replacement
- Pilot assembly cleaning
- Electronic ignition faults
- Same-day, parts on the truck
A standing pilot flame does one job beyond igniting the burner: it heats a thermocouple, a small probe that generates a tiny electrical current when hot. That current — around 25 to 30 millivolts — holds an electromagnet open inside the gas valve. If the pilot goes out, the current stops, the magnet drops, and the gas valve closes. It is elegant, entirely mechanical, and it is a safety system, not a nuisance.
So when the pilot dies the moment you release the control knob, the thermocouple is no longer producing enough current. That is the single most common gas water heater failure we attend.
Diagnosing by behaviour
| What happens | Most likely cause |
|---|---|
| Lights, burns while held, dies on release | Weak or failed thermocouple — or the knob was released too early |
| Pilot will not light at all | No gas at the valve, blocked pilot orifice, or air in the line after a gas outage |
| Pilot lights but is small, yellow or lifting | Partially blocked orifice, or restricted combustion air at the FVIR intake screen |
| Pilot stays lit but the burner never fires | Failed gas control valve or thermostat |
| Pilot goes out days or weeks later, repeatedly | Draft or downdraft in the flue, or an intake screen clogging with lint and dust |
| Electronic ignition clicks but no flame | Failed igniter, flame sensor fouling, or a control board fault |
Try this first
Relighting is safe to attempt yourself if you follow the sequence printed on the tank. Turn the control to OFF and wait a full five minutes for any accumulated gas to clear — this is not optional. Turn to PILOT, press and hold the knob, ignite the pilot with the piezo button or the lighter port, and keep holding for a full 30 to 60 seconds so the thermocouple reaches operating temperature. Then release slowly and turn the dial to ON.
If it will not stay lit after two attempts, stop. Repeated attempts accumulate unburned gas in the chamber. And if you smell gas at any point, leave the house, then call 911 and Xcel's gas emergency line from outside.
What we actually check
A thermocouple is a $20 part, but replacing it blindly fixes nothing if the real cause is a dirty pilot orifice or a flue problem that keeps snuffing the flame. Our sequence: measure the thermocouple's output under load (below roughly 18 millivolts it is done regardless of appearance), inspect and clean the pilot orifice and tube, vacuum the FVIR air-intake screen at the base of the tank, inspect the burner for lint and rust scale, verify gas pressure at the valve, and smoke-test the draft at the hood with the furnace and dryer running.
That last test is the one that separates a repair that lasts from one that fails again in three weeks. In tightly sealed Denver homes with powerful exhaust fans, a negative-pressure basement can pull the flue backward and blow the pilot out repeatedly, no matter how many thermocouples get fitted. The fix there is combustion air, not another part. More on that on our gas water heater repair page.
Thermocouple, thermopile or igniter?
Three different parts, often confused. A thermocouple powers only the pilot safety magnet on a basic standing-pilot unit. A thermopile is a bundle of thermocouples generating enough millivoltage to run an entire electronic gas control — used on units with a digital display and no external power. An igniter and flame sensor belong to electronically ignited models with no standing pilot at all. Which one your heater has determines both the part and the diagnostic approach, and it is why we ask for the model number when you call.
Frequently asked questions
Why won't my water heater pilot light stay lit?
Most commonly a failing thermocouple that no longer generates enough current to hold the gas valve open. Other causes: a dirty pilot orifice, a clogged air-intake screen, a downdraft in the flue, or a failed gas control valve.
How much does thermocouple replacement cost?
Typically $150–$280 in the Denver metro including the part, labor and a full pilot and burner clean. If the fault turns out to be the gas control valve instead, that runs $350–$650.
Can I replace a thermocouple myself?
It is a fairly simple mechanical swap, but it involves disconnecting gas connections at the control valve and correctly re-seating the pilot assembly. Any error there is a gas leak. If you are not comfortable leak-testing the connections afterward, call us.
How long should a thermocouple last?
Five to eight years typically. Failing much sooner points to a secondary cause — excess heat from a poorly adjusted pilot, or repeated snuffing from a draft problem.
My pilot keeps going out after storms or windy days. Why?
Downdraft. Wind across the flue termination reverses draft momentarily and blows the pilot out. A properly sized vent cap and correcting vent termination height usually fixes it; in some homes a conversion to a power-vent or direct-vent unit is the permanent answer.
Cold shower this morning? We can be out today.
Tell us what the heater is doing and we will give you a straight answer on the phone — repair, replace, or wait.