When a gas water heater stops working, the cause is usually one of four things: the pilot won’t light, the pilot won’t stay lit (most often a failing thermocouple or thermopile), a safety device has shut the burner down, or the heater isn’t getting the gas or air it needs. A few of these are safe to check yourself. The rest are repairs for a licensed technician.
This guide covers standard tank-style gas water heaters, including power-vent models. For tankless units, see our water heater error codes guide.
Stop First If You Smell Gas
- Leave the building now. Don’t try to light any appliance, flip a light switch, unplug anything, or use a phone inside.
- Call from outside or from a neighbor’s phone: your gas utility or 911. Puget Sound Energy: 1-888-225-5773. PG&E: 1-877-660-6789. SoCalGas: 1-800-427-2200. NW Natural customers can call 911.
- Don’t go back in until the utility or fire department says it is safe.
Carbon monoxide: headache, dizziness, weakness, nausea, sleepiness or confusion while the heater is running can mean CO. Get outside to fresh air and call 911. The CPSC recommends a CO alarm on every level of the home and outside sleeping areas.
Quick Symptom Lookup
| What you see | Most likely cause | Safe to try yourself? |
|---|---|---|
| Pilot won’t light | Gas off, air in the line, igniter not sparking, blocked air screen | Yes: check the gas valve and relight per the label |
| Pilot lights, then goes out | Thermocouple or thermopile, weak pilot, drafts or poor combustion air | One relight. If it keeps happening, call |
| Status light blinking | The gas control is reporting a fault | Read the code first (table below) |
| Pilot lit, burner never fires | Thermostat set low, tripped high-limit, failed gas control | Check the setting only |
| Yellow, lazy or sooty flame | Incomplete combustion: dirty burner, blocked air or vent | No: turn it off and call |
| Rumbling or popping | Sediment on the tank bottom | Flushing, if your manual allows it |
| Sizzling at startup | Condensation dripping on the burner. Normal while the tank is cold | Nothing to fix |
| Hot water runs out fast | Thermostat, sediment, broken dip tube, undersized heater | Check the setting |
| Power-vent unit does nothing | No power to the blower, blocked vent, igniter lockout | Check the plug and breaker |
Step 1: Confirm Gas and Read the Label
Is gas reaching the heater?
The shutoff valve on the gas line next to the heater should be open, with the handle running in line with the pipe. If your stove or furnace is also out, the problem is the gas supply, not the water heater, so call your gas utility.
What does your label say?
Lighting instructions are printed on the heater itself, and waiting times differ by brand: A.O. Smith and State say 10 minutes between lighting attempts, Rheem says 5 minutes to let gas clear, and Bradford White says 2 minutes after the control is pressurized.
Rule of thumb: follow the label on your heater over any general guide, including this one.
Pilot Light Won’t Light
Most newer gas water heaters light with a push-button igniter. On A.O. Smith and State models, the manufacturer’s instructions work like this:
Wait at least 10 minutes since the last attempt.
Press and hold the gas control knob.
Click the igniter about once per second for up to 90 seconds, until the pilot lights or the status light starts flashing.
Stop immediately if you smell gas.
If you can’t get it to light
Gas was just turned back on
Air in the line can take a few tries to clear after gas work or a meter change.
No spark
In a dark room you should see a spark at the pilot when you press the igniter. No spark usually means a failed igniter or loose wire.
Blocked air screen
Lint, dust and pet hair at the base of the heater can starve the pilot of air. This is common in garages and laundry rooms.
If it still won’t light after two or three tries, stop and call us. More attempts won’t fix a faulty heater.
Pilot Keeps Going Out: The Thermocouple and Thermopile
Besides lighting the burner, the pilot flame heats a small sensor that tells the gas control a flame is present. On older heaters that sensor is a thermocouple. On most newer heaters it’s a thermopile, which also powers the electronic control. If the sensor is weak or the flame isn’t hitting it properly, the control shuts off the gas, and the pilot goes out.
A.O. Smith’s manual says that if you can see the pilot but the status light never starts blinking, the pilot may not be heating the thermopile enough, the thermopile may be defective, or a wiring connector may be loose.
It isn’t always a bad part
State’s service handbook also lists these causes:
Combustion air. Not enough makeup air in the room, or a blocked air screen or flame arrestor.
Venting. A vent that isn’t drawing properly.
Depressurization. Exhaust fans, a dryer or an attic fan pulling air out of the house.
Drafts. Wind or moving air at the heater.
Can you replace a thermocouple yourself?
On an older open-burner heater, many homeowners do. Most newer heaters are flammable vapor ignition resistant (FVIR) designs with a sealed burner compartment and a thermopile. The right part and procedure depend on the exact model, and a mistake changes how the heater burns gas, so we recommend having a technician do it. If the pilot keeps going out after a new sensor, the cause is usually combustion air or venting, and that’s what we look for next.
What the Blinking Status Light Means
Many newer gas heaters use an electronic gas control with a status light. This is the code table from A.O. Smith’s and State’s manuals. Other brands use similar controls, but check your heater’s label before relying on it.
| Status light | What it means | What to do |
|---|---|---|
| Off | Pilot isn’t lit, or the thermopile has not warmed up yet | Wait 10 minutes and relight per the label |
| Solid on | Pilot recently went out and the thermopile is cooling | Wait 10 minutes before relighting |
| 1 flash every 3 seconds | Normal operation | Nothing |
| 2 flashes | Pilot is lit but thermopile output is low | Call: weak pilot, wiring or a failing thermopile |
| 4 flashes | Water got too hot and the high-limit shut the heater down | Call. If it repeats, the manual calls for replacing the gas control |
| 5 flashes | Temperature sensor fault | Call: gas control replacement |
| 7 flashes | Gas control failure | Call: gas control replacement |
| 8 flashes | Control still sees a flame with the knob off | Turn off the gas at the shutoff valve and call |
Bradford White uses different controls with color-coded lights. On its newer controls, for example, 2 yellow flashes mean low thermopile voltage and 4 red flashes mean the temperature limit was reached. Codes vary by model, so check the label or our error codes guide.
Pilot Is Lit but the Burner Won’t Fire
Check the temperature dial first. It may have been turned down or bumped to “vacation.” If it’s set normally and the burner still won’t come on, one of the heater’s safety devices may have shut it down.
High-limit (energy cut-off)
Built into the gas control. It shuts the heater off if the water overheats. Rheem describes it as a single-use device, so a trip means a technician replaces the gas control.
Thermal switch / flame rollout
Trips when flames or heat escape the combustion chamber, usually from poor combustion air, blocked venting or flammable vapors. Some A.O. Smith and State switches can be reset, but only after the cause is fixed. Rheem’s thermal release device isn’t homeowner-resettable. If the switch trips more than once, the problem is combustion air or venting, not the switch.
Flammable vapor lockout
Gasoline, paint, thinners and solvents stored near the heater can trigger it. Rheem models with a vapor sensor lock out until serviced. A.O. Smith and State say a heater exposed to a vapor ignition shows discoloration on the flame arrestor and needs to be replaced. Keep those products away from the heater.
What the Flame Should Look Like
Normal
A soft blue flame. According to A.O. Smith, blue flames with yellow inner cones are fine, and some yellow tipping is normal on propane.
Turn it off and call
An all-yellow, lazy or sooty flame means incomplete combustion from a dirty burner or not enough air. Rheem lists blocked vents and combustion-air openings among the causes. Poor combustion can produce carbon monoxide.
Sizzling, Rumbling and Popping
Sizzling at startup: normal
Condensation drips onto the burner while the tank is cold. A.O. Smith, Rheem and Bradford White all call it normal, and it stops once the water heats up.
Rumbling or popping: sediment
Sediment and scale on the tank bottom trap water, which flashes to steam as the burner heats it. Flushing helps.
Manufacturers differ on how often to flush: A.O. Smith’s manual calls for draining and flushing every six months, while Rheem and Bradford White suggest draining a small amount every month. Follow your heater’s manual. More on diagnosing a noisy water heater.
Hot Water Runs Out Fast
A gas heater that works but runs out quickly usually has one of these problems:
Thermostat set too low
The CPSC and the Department of Energy recommend 120°F for most homes. At 140°F, a scald burn can happen in seconds. More on the right temperature setting.
Sediment
Buildup takes up space at the bottom of the tank.
Broken dip tube
A broken or melted dip tube lets incoming cold water mix with the hot water at the top.
Heater is too small
The Department of Energy says to size a tank by its first-hour rating, matched to your busiest hour of hot water use.
Power-Vent Gas Water Heaters
A power-vent heater has an electric blower on top and plugs into a 120-volt outlet. Many have no standing pilot. A hot-surface igniter tries to light a few times and then locks out.
If it does nothing at all
- Check that it’s plugged in and the breaker is on.
- Check the outside vent openings for snow, leaves or nests. A pressure switch shuts the heater down if the intake or exhaust is blocked.
- Read the fault code on the unit. Power-vent codes are model-specific.
Repair or Replace?
Usually worth repairing
A thermocouple, thermopile, igniter or gas control on a heater that is otherwise sound.
Time to replace
Water leaking from the tank itself, which can’t be repaired. ENERGY STAR also suggests considering replacement after 10 years, or when you see leaks, rust in the water or hear rumbling. See water heater replacement.
Still No Hot Water? We Can Help Today
Fast Water Heater Co. has repaired and replaced gas water heaters across Washington, Oregon and California since 1986. Call by noon for same-day water heater repair in most areas, with free estimates and no trip fees.
Gas Water Heater Troubleshooting FAQ
Why does my water heater pilot light keep going out?
The most common cause is a weak or failing thermocouple or thermopile, the sensor the pilot flame heats to keep the gas valve open. Drafts, a blocked air screen, not enough combustion air, venting problems and exhaust fans pulling air from the house can also put the pilot out. If it goes out more than once, have it checked rather than relighting it again.
How do I know if the thermocouple is bad?
The usual sign is a pilot that lights while you hold the control knob but goes out when you let go. On newer heaters with a status light, 2 flashes means the thermopile output is low. A technician can test the sensor’s output to confirm before replacing it.
What do the blinking lights on my water heater mean?
On A.O. Smith and State gas heaters with an electronic control, one flash every three seconds is normal. 2 flashes means low thermopile output, 4 means the high-temperature limit tripped, 5 is a sensor fault, 7 is a gas control failure, and 8 means the control still detects a flame with the knob off. Bradford White uses different color-coded codes.
Is it safe to relight a water heater pilot myself?
Yes, if you don’t smell gas and you follow the lighting instructions on the heater’s label, including the waiting time. Stop and call for service if you smell gas, if the pilot won’t stay lit after a couple of tries, or if the flame is yellow.
How long do gas water heaters last?
The Department of Energy puts the typical life of a storage tank water heater at 10 to 15 years. ENERGY STAR suggests considering replacement after 10 years, especially if you see leaks, rust in the water or hear rumbling.
Sources
- A.O. Smith: How to light a pilot light
- A.O. Smith atmospheric gas water heater manual
- State water heaters service handbook
- Rheem gas water heater use and care manual
- Bradford White gas water heater manual
- CPSC Carbon Monoxide Information Center
- CPSC: Tap water scalds
- U.S. Department of Energy: Sizing a new water heater
- ENERGY STAR: When should you replace your water heater?
- Gas leak safety: Puget Sound Energy
- Gas leak safety: NW Natural
- Gas leak safety: PG&E
- Gas leak safety: SoCalGas
Last reviewed October 2026.
