Gas Water Heater Repair: Pilot Lights, Thermocouples, and Five Other Common Fixes

Match your symptom to the likely cause

Thermocouple replacement (the most common gas heater repair)

The thermocouple is a small metal probe positioned in the pilot flame. When the flame heats it, it generates a tiny voltage that holds the pilot gas valve open. When the thermocouple weakens or fails — typical life 6–10 years — the voltage drops below the threshold, the gas valve closes, and the pilot goes out.

Symptom: pilot lights when you hold the button down, then goes out within 30 seconds of releasing. Diagnostic confirmation: hold the pilot button for the full 60 seconds the manufacturer specifies. If it still goes out, the thermocouple is the issue. If it stays lit while you hold but goes out a few minutes later, also thermocouple.

The part costs $10–$20 at any hardware store. The labor to swap it runs 20–40 minutes. Plumber pricing for the service call typically lands $120–$280 total. A DIY fix is very reasonable — there are five connection points and the gas supply to the heater is shut off during the swap.

Gas control valve failure

The gas control valve is the brick-shaped block on the front of the heater that the supply pipe attaches to. It controls gas flow to both the pilot and the burner, regulates temperature via an internal thermostat, and has the temperature dial on its face. When it fails, the unit either does not light at all, has erratic temperature, or fails to shut off the gas supply (a safety concern).

Symptoms: pilot works correctly but the main burner does not light when there is hot water demand. Or temperature is uncontrolled — water comes out either far below or far above setpoint. Or the gas keeps flowing to the burner even after the thermostat should have shut it off (rare but dangerous).

Replacement is plumber work — the gas supply must be properly disconnected and reconnected with thread compound and leak-tested. Cost: $320–$650 installed.

One specific note: Honeywell-made gas control valves (used on many residential units 2010–2018) had a known issue with thermopile failures that mimicked gas valve failure. If your unit is from that range and shows gas valve symptoms, the thermopile assembly is the cheaper first thing to replace ($80–$150).

Dip tube failure (the diagnosis nobody guesses correctly)

The dip tube is a long plastic pipe that runs from the cold-water inlet at the top of the tank down to within 8–10 inches of the bottom. Cold incoming water travels down the tube to the bottom of the tank, gets heated, rises, and exits via the hot-out port at the top.

When the dip tube cracks, breaks, or disintegrates (some 1993–1997 units had defective dip tubes that crumbled), cold water enters at the top of the tank and immediately mixes with the hot water at the outlet. The result: lukewarm water at all taps, even when the burner is firing correctly.

Symptom: hot water that is consistently warm but never actually hot, no matter how high you set the temperature. The burner is firing, the gas valve is working, the pilot is lit — but the output is 95°F instead of 120°F. Diagnostic: open the cold-water valve and feel temperature directly out of the dip tube fitting at the top.

Replacement is a 30-minute job for a plumber. The new dip tube ($30–$80) is a long plastic tube that drops in through the cold-inlet fitting after the unit is drained partway. Total cost installed: $80–$220.

Sediment causing popping noises

Hard water deposits calcium carbonate on the bottom of the tank where the burner is. As the deposit grows, it creates pockets where water trapped under the sediment briefly steams when the burner fires — that "pop" or "rumble" you hear is essentially small steam explosions under the sediment layer.

This does not damage the unit immediately but reduces efficiency significantly (the sediment insulates the burner from the water) and accelerates burner element wear. Over a few years, the burner pan can warp from uneven heating.

The fix is a complete tank flush — drain the unit fully, briefly open the cold supply with the drain still open to flush out sediment, repeat until water runs clear. A 45-minute job that any homeowner can do. Plumber pricing for a flush: $180–$280.

Frequency: annually in hard-water areas (above 7 grains per gallon), every 18 months in moderate water. If you have never flushed and the unit is 5+ years old, the first flush sometimes triggers a leak in a previously-sealed corroded area. The risk is small but real.

Anode rod (the maintenance everyone skips)

The anode rod is a sacrificial magnesium or aluminum rod that hangs into the tank from the top. It corrodes preferentially to the steel tank, protecting the tank from rust through. Over 3–5 years it depletes to a thin wire and stops protecting; the next phase of corrosion is the tank itself.

Symptom: rusty hot water (the unit is starting to corrode), or the tank simply failing through rust-out in years 8–12 instead of 12–15.

Inspection: every 3–4 years, remove the anode rod (large hex head on top of the tank) and look at it. Healthy anode is the original 3/4" diameter; depleted anode is a thin twisted wire. Replacement anode: $25–$45. Installation difficulty: the hex head requires a 1-1/16" socket and significant torque to break loose. Many homeowners cannot manage this without an impact driver.

Plumber service for anode inspection/replacement: $180–$320. This is the highest-ROI maintenance you can do on a tank water heater. A $200 anode swap at year 4 typically adds 3–5 years to the tank's life.

T&P valve dripping

The temperature-and-pressure relief valve discharges water if the tank exceeds 150 PSI or 210°F. Its discharge tube routes to within 6 inches of the floor, so any water from it lands in a visible place. Discharges fall into three categories:

• Steady continuous drip: thermal expansion in a closed system. You need an expansion tank installed on the cold supply ($150–$300). The drip is the symptom; the closed plumbing system is the cause.
• Occasional small drip: failing T&P valve. Replace it ($150–$280 installed). The valve is a wear part — typical life 8–12 years.
• Steady stream during recovery: failed thermostat allowing overheating. The T&P is doing its job; the gas control valve needs replacement.

Never cap a dripping T&P discharge. The valve is the only thing preventing the tank from exploding under runaway pressure conditions. Find the cause; do not silence the symptom.

Which of these can you DIY

DIY-reasonable for a moderate-handy homeowner:

• Thermocouple replacement.
• Annual tank flush.
• Anode rod inspection (assuming you have the tools to break the hex head loose).
• Dip tube replacement.

Plumber territory:

• Gas control valve replacement (gas line disconnection requires proper procedure).
• T&P valve replacement on units with corroded threads.
• Any work that involves gas leak-testing (which is most of the above for newer high-pressure systems).

When repair stops making sense

For gas tank water heaters, the financial decision point is roughly:

• Year 1–7: almost always repair. Unit has many years left and parts cost is small relative to replacement.
• Year 8–10: repair single-component issues under $400. Replace if the failure is the gas valve and the tank is already showing rust signs.
• Year 11–13: only repair if the issue is cheap and the rest of the unit is clean. Most tank failures in this range are tank-rust-through (not repairable).
• Year 14+: any meaningful repair cost is the moment to replace. You are buying months of remaining life at high $/month cost.

If you have to replace, look at heat-pump and condensing options seriously — operating costs have changed significantly in the last 5 years and you may not want another atmospheric-vent gas tank.