Cold shower at 6 a.m. is bad enough. Standing in the basement at 6:15 a.m. clicking the igniter button on a water heater that refuses to fire is worse. A water heater not lighting almost always traces back to one of five issues — and four of them are DIY fixable in under 30 minutes with no special tools. The fifth requires a plumber, but knowing which one you have saves you from paying $250 just to be told you have a bad thermocouple.
Step Zero: Confirm It Is the Water Heater
Before pulling anything apart, check the obvious. Is the gas on at the meter or propane tank? If your stove will not light either, you have a supply problem, not a heater problem. Open a stovetop burner and see if it ignites. If your home runs on natural gas and there has been recent work in the neighborhood, the utility may have shut off your supply temporarily.
Next, check the temperature dial on the front of the tank. Sometimes it gets bumped to the “Vacation” or “Pilot” setting accidentally. Turn it back to “Hot” or to a temperature between 120 and 140 degrees Fahrenheit.
Pilot Light Will Not Stay Lit
This is by far the most common complaint and almost always points to one part: the thermocouple. The thermocouple is a thin metal rod about 3 inches long sitting next to the pilot flame. When the flame heats it, it generates a tiny electrical signal that tells the gas valve to stay open. When it fails, the gas valve shuts off as soon as you release the pilot button.
Replacement runs $10 for the part at any home center and takes about 20 minutes. You need an adjustable wrench and ideally a pair of needle-nose pliers.
- Shut off the gas supply at the inlet valve (turn the lever perpendicular to the pipe).
- Disconnect the thermocouple nut from the gas control valve. It is the smallest of the three or four connections on the valve.
- Slide the thermocouple out of its mounting bracket near the burner.
- Install the new thermocouple, positioning the tip so the pilot flame will wrap around the top 1/2 inch of it.
- Hand-tighten the connection nut, then snug another 1/8 turn with the wrench — no more, or you will strip the soft brass.
Relight the pilot following the instructions on the heater label. Hold the pilot button down for 60 seconds after the flame catches, then release.
Pilot Will Not Light At All
If the pilot will not ignite when you press the igniter and hold the gas button, the issue is usually one of these:
- Dirty or clogged pilot tube — debris blocks the gas flow to the pilot orifice
- Faulty igniter (the piezo electric clicker)
- Air in the gas line from recent service work
- Closed gas supply valve
For a clogged pilot tube, shut off the gas, remove the tube (usually held by one or two screws), and blow through it with compressed air. A can of compressed air costs $5 and clears spider webs and dust that have built up over the years.
If the piezo igniter clicks but produces no spark, you can light the pilot manually with a long fireplace match while holding the gas button down. Replace the igniter assembly when convenient — about $25 for the part.
Pilot Lights But the Burner Will Not Fire
If the pilot stays lit but the main burner does not ignite when the thermostat calls for heat, you are dealing with either a stuck gas valve or a wiring issue with the thermal switch. The thermal switch is a safety device on the side of the heater that trips when the unit overheats.
Try pressing the small red or black reset button on the thermal switch. If it clicks back in, run the heater and see if it cycles normally. If it trips again within an hour, you have a venting problem — usually a blocked flue or a downdraft pushing exhaust back into the unit.
A bad gas valve is the more expensive diagnosis. Replacements run $150 to $300 in parts plus labor if you hire a plumber. Some homeowners DIY this, but gas connections are not the place to learn — if you are not 100 percent confident, hire a pro.
Electric Water Heaters That Will Not Heat
Electric water heaters do not have a pilot, but they have their own version of the same problem. If you hear nothing from the unit and have no hot water, check:
- Circuit breaker — should be on. Reset by flipping fully off then back on.
- High-limit reset switch — small red button behind the upper access panel. Press it firmly.
- Heating elements — test with a multimeter set to ohms. Reading should be 10 to 30 ohms. Infinite reading means a burned-out element.
Replacement elements cost $15 to $25 each and screw into the tank. Drain the tank first, shut off the breaker, and have a wrench plus the right element wrench (a 1 1/2-inch socket works).
When the Whole Unit Is Just Old
Tank-style water heaters last 8 to 12 years. After 10 years, the tank itself can develop pinhole leaks, the burner assembly corrodes, and repair after repair starts to stack up. If your heater is over 10 years old and you are looking at a $300 repair, replace the whole unit instead.
A standard 40-gallon natural gas water heater runs $500 to $800 at Home Depot. Installation by a plumber typically adds $400 to $700, depending on whether the venting and gas lines need updating. Tankless conversion costs more upfront ($1,800 to $3,500 installed) but saves 20 to 30 percent on gas use long term.
Safety Notes That Matter
Two things to take seriously. First, if you smell gas at any point during troubleshooting, stop, leave the building, and call the gas utility — they come for free 24/7. Second, never bypass safety switches or wedge open the gas valve to keep a faulty unit running. The safeties exist because gas water heaters can produce carbon monoxide when something is wrong with combustion or venting.
Install a carbon monoxide detector on every floor of your home if you have not already. The $30 First Alert combo CO and smoke alarm is genuinely the cheapest piece of safety equipment you can own.
