Gas Water Heater: No Hot Water — Pilot, Thermocouple, and the Lockout Nobody Mentions

A gas water heater with no hot water is usually one of three things: the pilot is out, the thermocouple will not hold the pilot lit, or the gas control valve has failed. Newer units add a fourth: a flammable-vapor sensor that locks the whole heater out after a vapor event or an overheat. Work them in that order, and stop the moment you smell gas.

1. Look through the sight glass

There is a small window near the bottom of the tank, at the burner compartment. Look in. You are checking whether the pilot flame is lit and whether the main burner fires when the thermostat calls.

• Pilot out, no flame: relight it. The lighting instructions are printed on the side of the tank — follow them exactly, including the wait period before you try. Modern units use a piezo clicker instead of a match, and the knob sequence matters.
• Pilot lit but the burner never fires: the thermostat is not calling, or the control valve is not opening. Go to step 3.
• Pilot lit, burner fires, water still cold: look at the flame. A good flame is blue with maybe a little orange at the tips. A lazy yellow flame or soot is a venting problem — stop and read the STOP line at the bottom.

2. Lights but will not stay lit — the thermocouple

You hold the knob down, the pilot lights, you release it after the prescribed time, and the pilot goes out. That is the classic thermocouple failure.

The thermocouple is a small probe sitting in the pilot flame. It generates a few millivolts when heated, and that tiny current holds the gas valve open. When it gets dirty, bends away from the flame, or just wears out, it stops generating enough and the valve closes.

• Check that the probe is actually sitting in the flame. Sometimes it has shifted.
• Clean the tip with fine steel wool and try again.
• If it still will not hold, replace it. The part is $10–$25.

On a millivolt system — one with no electrical connection at all — the equivalent part is a thermopile, and it does the same job with more output. Same test, different part.

Hired, a thermocouple replacement runs $150–$250. That is mostly labor, because the burner assembly has to come apart.

3. Pilot stays lit but the burner never fires

The pilot is burning, the thermostat is set higher than the tank temperature, and the main burner will not light. That points at the gas control valve or the thermostat sensor.

• Confirm the gas is on at the shutoff valve on the supply line. Sounds simple; it is the first thing to check.
• Check whether other gas appliances work. If the range and the furnace are also dead, call the gas company.
• If everything else works, the control valve is the suspect. These run $150–$250 for the part, and $400–$700 installed.

On a tank past 10 years, a failed control valve is usually a replace-the-heater decision. The valve plus labor puts you well into the price of a new unit, and the new one comes with a warranty and a modern burner assembly.

4. The lockout nobody mentions

Newer gas water heaters have a flammable-vapor sensor, sometimes called an FVIR sensor or a thermal switch, near the burner compartment. If gasoline, paint thinner, or solvent vapors get pulled into the intake, the sensor trips and locks the unit out. It can also trip after an overheat event or a blocked intake.

• Check whether you recently used or stored paint, gasoline, or solvent near the heater — a garage install is the usual place for this.
• Some units reset with a button or a power cycle per the label. Others require the sensor to be replaced.
• Ventilate the area and remove the source before you try to reset anything.

STOP: If the lockout happened because of a vapor event, do not just reset the sensor and walk away. Find and remove the source of the vapors. The sensor is there because the heater’s air intake pulled them in.

5. Venting trouble

Soot at the draft hood, a lazy yellow flame, condensation running down the flue, or burn marks around the draft hood are all signs of a venting failure. That means combustion products are not going up the flue the way they should, and carbon monoxide can end up in the house.

STOP: Soot, a yellow flame, or a backdrafting draft hood means turn the heater off at the gas valve and call a pro. Do not relight it, do not “see if it clears.” Carbon monoxide is not a nuisance.

STOP: If you smell gas at any point, get out of the house and call the gas company from outside. Do not flip switches, do not use a phone inside, do not hunt for the leak with a flame.

2026 price ranges

Repair or replace

Under 10 years old and the tank is sound, repair. Thermocouple or sensor, fix it. Past 10 years with a failed control valve, price the new heater first. Look at the tank bottom and the fittings for rust streaks before you decide — a tank that is starting to weep is not worth a $500 valve.

Check the anode rod once a year if you want the tank to last. A $25 rod is what stands between you and a rusted-out tank at year twelve.