Gas Water Heater Won't Ignite? 9 Causes and How to Fix Each

The Short Answer

Most gas water heater ignition failures trace back to one of nine problems: a gas supply issue, a tripped safety reset, a triggered flammable vapor sensor, a clogged pilot tube, a bad thermocouple or thermopile, a failed igniter, a dirty burner assembly, a bad gas control valve, or a blocked exhaust vent. All nine are covered in the order you should check them, with status light codes and step-by-step fixes for each.

Safety First

Gas appliances require caution. Before you do anything:

• If you smell gas — stop immediately. Leave the house, don't flip any switches, and call your gas company from outside.
• Turn the gas control knob to "Pilot" or "Off" and wait at least 5 minutes before working on the unit. This lets any residual gas dissipate.
• Make sure the area is well ventilated. Keep flammable materials away.
• If you're uncomfortable at any point, call a licensed plumber. These steps cover diagnosis and the common fixes, not every situation — and there's no shame in calling a pro.

Standing Pilot vs. Electronic Ignition: Which Type Do You Have?

Before you troubleshoot, you need to know which ignition system your unit uses. The fixes are different.

Standing Pilot (Older and Common)

A standing pilot keeps a small flame burning continuously. When your water heater calls for heat, that flame ignites the main burner. To light it, you physically press and hold the gas control knob and use a built-in igniter button or a long lighter.

How to tell: Look through the small viewing window at the bottom of your water heater. If you see a small blue flame burning even when the unit isn't actively heating, you have a standing pilot. If the viewing window is dark, either the pilot is out or you have electronic ignition.

See our step-by-step guide on how to relight a water heater pilot light safely if that's your starting point.

Electronic Ignition (Newer Units)

Electronic ignition creates a spark on demand. There's no always-on pilot flame. When your thermostat calls for heat, you'll hear clicking as the igniter fires to light the burner. Most units made in the last 15 years use some form of electronic ignition.

How to tell: If you hear clicking when you turn up the temperature knob, you have electronic ignition. If there's no viewing window at all, or the window is always dark, same conclusion.

Electronic ignition units rely on a thermopile rather than a thermocouple, an important distinction covered in section 5 below.

Reading Your Status Light

Most gas water heaters made after 2000 have a small LED indicator on the gas control valve. It blinks in patterns to communicate fault codes. Before you start disassembling anything, count the flashes.

Blink patterns vary by brand and gas valve manufacturer, but here are the most common codes across Honeywell/Resideo-valved units (which cover a large share of Rheem, A.O. Smith, Bradford White, State, and others):

• 1 flash every 3 seconds: Normal operation — no fault
• 2 flashes: Thermopile voltage too low (common on electronic ignition units)
• 3 flashes: Pressure switch or draft sensor open (power-vent models)
• 4 flashes: High temperature limit exceeded — ECO tripped
• 5 flashes: Exceeded high temperature on dual thermostat systems
• 7 flashes: Gas valve internal fault
• Rapid continuous flashing: Gas valve failure — call a professional
• No light at all: No power reaching the valve, or the valve itself has failed

Important: These codes apply to many common units but are not universal. If your blink pattern isn't listed here, check your owner's manual or search your model number with "status light codes." The label on your gas control valve sometimes lists the codes directly.

1. Gas Supply

Start with the obvious. It gets overlooked more often than you'd think.

What to Check

• Shutoff valve: The gas valve on the supply line leading to your water heater should be fully open. The handle runs parallel to the pipe when open; perpendicular means off.
• Other gas appliances: Light a burner on your stove, or check your furnace. If nothing gas-powered in the house works, the problem is upstream, at the meter or with your gas provider.
• Main meter valve: Confirm the meter valve hasn't been shut off (sometimes during utility work or a previous emergency).
• Propane tank (if applicable): Check gauge and swap tanks if necessary. A propane tank can drop low enough to prevent ignition even if it isn't completely empty.
• Low gas pressure: In rare cases, supply pressure may be too low to support ignition without being fully off. This is more common right after service work or in homes at the end of a long gas line run. A plumber or your gas utility can check pressure with a manometer.

2. High-Limit Reset Button (ECO)

This is the single most overlooked quick fix. Most gas water heaters have a high-limit reset button (also called a thermal switch or ECO, or Energy Cut-Off) that trips automatically when the unit overheats or detects a problem. Once tripped, the unit won't ignite until it's manually reset.

How to Find and Reset It

• Look for a small button on the gas control valve or behind a rubber plug near the thermostat. On some models it's accessible through a small hole that requires a thin probe (like a pen tip) to reach.
• Press firmly until you feel or hear a click.
• Attempt to relight or restart the unit.

If the ECO trips again after resetting, don't keep pressing it. If the high-limit keeps tripping, the water is genuinely overheating. Check your temperature setting (the recommended range is 120°F for most households; see our guide on what temperature your water heater should be set to) or there's a failing thermostat or gas valve that needs replacement.

3. Flammable Vapor Sensor (FVIR)

Since 2005, all residential gas water heaters sold in the US are required to be FVIR-compliant (Flammable Vapor Ignition Resistant). These units include a thermal fuse or vapor sensor at the base that trips if flammable vapors (gasoline, paint thinner, solvents) accumulate near the unit and ignite.

When the FVIR sensor trips, the unit locks out completely and won't ignite. From the outside it looks exactly like any other ignition failure, but the fix is completely different.

Symptoms

• The unit suddenly stopped working with no obvious cause
• You store flammable liquids in the same room (garage, utility room)
• There was a recent spill of gasoline or solvent nearby
• The reset button and pilot relighting attempts have no effect

How to Reset It

1. Turn off the gas supply and open doors and windows to thoroughly ventilate the space. Wait at least 30 minutes. This step matters.
2. Remove any flammable liquids from the room permanently. The FVIR system is designed to lock out until the hazard is gone.
3. Consult your owner's manual for your specific reset procedure. On some units the FVIR fuse is a one-time thermal link that requires replacement; on others it's a resettable button.
4. After resetting (or replacing the fuse), attempt to relight the unit.

Note: If the FVIR sensor keeps tripping, the underlying issue is storage of flammable materials near the water heater, not the water heater itself. Many jurisdictions require a minimum clearance between gas appliances and flammable storage.

4. Dirty or Clogged Pilot Tube

(Standing pilot units only)

The pilot tube is a small metal line that delivers gas to the pilot light. Over time, dust, debris, spiderwebs, and mineral deposits can partially or fully block it, preventing enough gas from reaching the pilot to ignite or stay lit.

Symptoms

• Pilot won't light at all, even though gas is flowing elsewhere
• Pilot lights weakly or with a yellow/orange flame instead of blue
• Pilot lights briefly then immediately goes out before you release the knob

How to Fix It

1. Turn the gas control knob to "Off" and wait 5 minutes.
2. Remove the access panel at the bottom of the water heater to expose the burner assembly.
3. Locate the thin pilot tube (it runs from the gas control valve to the pilot burner).
4. Use a thin needle or sewing pin to gently clear the opening of the tube. Don't enlarge the hole.
5. Follow up with a short burst of canned compressed air to clear any remaining debris.
6. Reassemble and attempt to relight.

A clean pilot should produce a small, steady blue flame with the tip just touching the thermocouple. If the flame is yellow, orange, or flickering, the tube may still be partially blocked or the thermocouple alignment is off.

5. Thermocouple or Thermopile Failure

This is one of the most common causes of ignition failure, and the fix differs depending on which device your unit has.

Thermocouple (Standing Pilot Units)

A thermocouple is a safety device that senses whether the pilot flame is burning. It generates a small electrical signal (~25 millivolts) when heated by the pilot flame, which tells the gas valve to stay open. If the thermocouple is dirty, misaligned, or failed, the gas valve closes as a safety precaution.

Telltale symptom: Pilot lights while you hold the knob down, but goes out immediately when you release it — even after holding for 60 full seconds.

For a full walkthrough of testing and replacing a thermocouple, see our thermocouple replacement guide. The short version:

• Clean it first: Use fine sandpaper or steel wool to gently clean the thermocouple tip. Oxidation and soot can prevent it from sensing heat. This fixes the problem more often than people expect.
• Check alignment: The thermocouple tip needs to sit directly in the pilot flame. If it's been shifted, carefully bend it back into position.
• Replace it: Thermocouples cost $10–15 at any hardware store and take about 15 minutes to swap out. Unscrew the old one from the gas valve, remove from the bracket, install the new one in the same position.

You can also test a thermocouple with a multimeter: disconnect it from the gas valve and heat the tip with a lighter. A good thermocouple reads 25 mV or higher. Below 20 mV means it should be replaced.

Thermopile (Electronic Ignition Units)

Newer water heaters use a thermopile instead of a thermocouple. It looks similar but works differently. A thermopile generates much more voltage (~750 millivolts vs. 25 mV), enough to power the electronic gas valve, the digital thermostat, and the control display, not just hold the valve open.

Telltale symptom: The whole unit seems completely dead — no display, no response to controls, nothing. (A bad thermocouple just loses the pilot; a bad thermopile kills the whole unit's electronics.) Your status light may show 2 flashes.

• Test with a multimeter: Disconnect the thermopile from the gas valve and heat it with a lighter. A healthy thermopile reads 300–750 mV. Below 300 mV means it's weak or failed.
• Replacement cost: $20–40 at hardware stores or online. The procedure is similar to a thermocouple: unscrew from the valve, replace, reconnect.
• Also check the pilot: A thermopile that reads low but isn't failed sometimes just has a weak or misaligned pilot flame. Make sure the pilot is burning strong and directly heating the thermopile tip before replacing it.

6. Faulty Igniter (Piezo or Electronic)

The igniter creates the spark that lights the pilot or burner. If it fails, you'll have gas but nothing to light it.

Types

• Piezo igniter: A manual button that generates a mechanical spark, no batteries or electricity required. Common on older standing-pilot units.
• Electronic igniter: Battery-powered or line-powered, fires automatically when the thermostat calls for heat. You'll hear clicking. Common on newer units.

How to Test It

1. Darken the room and look through the viewing window at the bottom of the water heater.
2. Press the igniter button (or turn up the thermostat on electronic units).
3. You should see a visible blue-white spark near the pilot burner.
4. No spark = igniter issue.

How to Fix It

• Check the battery first: Battery-powered igniters (usually a AA or D cell near the igniter button) are the most common missed fix. Replace it before anything else.
• Clean the igniter tip: Use a soft brush to remove dust and carbon buildup from the tip.
• Check wire connections: Loose or corroded igniter wires are a frequent culprit. Reseat connectors and look for corrosion or breaks.
• Replace the igniter: Replacement igniters cost $15–35 and are available for most models. Make sure you match the correct part number for your water heater.

7. Clogged Burner Assembly or Flame Arrestor

The burner assembly is where the main flame burns to heat the water. The flame arrestor is a fine mesh screen at the base that satisfies FVIR requirements, but it also catches dust, lint, and pet hair over time.

Symptoms

• Pilot lights fine, but main burner won't ignite
• Burner fires briefly then shuts off
• Weak or uneven flame pattern across the burner
• Soot or black marks around the burner area or access panel

How to Fix It

1. Turn off gas and let the unit cool completely.
2. Remove the access panel and burner assembly (usually a few screws).
3. Vacuum the flame arrestor screen at the base of the unit. Use a brush to work out embedded debris.
4. Use compressed air or a soft brush to clean the burner ports (the small holes where the main flame comes out).
5. Inspect the burner for cracks, rust, or heavy corrosion. A damaged burner needs replacement.
6. Reassemble and test.

A clogged flame arrestor is especially common in homes with pets, high-dust environments, or units installed in garages or utility rooms with poor airflow.

8. Bad Gas Control Valve

The gas control valve is the central control unit of your water heater. It regulates gas flow, manages the thermostat, and handles safety shutoffs. When it fails internally, gas can't reach the burner properly, even if the pilot lights.

Symptoms

• Pilot lights but main burner never fires
• No clicking sound when you increase the temperature setting
• Temperature control feels unresponsive
• 7 flashes on the status light
• Rapid continuous flashing on the status light

What to Do

Gas control valve replacement is not a DIY repair. It requires disconnecting gas lines, and an improper installation is a serious safety risk. Call a licensed plumber. Expect $150–250 for the valve plus labor.

Before authorizing that repair: if your water heater is over 10 years old, a failed gas valve is often the point where replacement makes more financial sense than repair. Use our guide to decide whether to repair or replace, and check current replacement costs to compare.

9. Blocked or Damaged Exhaust Vent

Gas water heaters exhaust combustion gases (including carbon monoxide) through a vent pipe. If that vent is blocked, disconnected, or improperly installed, the unit may refuse to ignite or shut itself down as a safety measure. Power-vent models have a pressure switch that detects vent blockages and prevents ignition directly.

Common Causes

• Bird nests or animal debris in the vent cap or pipe
• Disconnected or sagging vent sections (especially after attic or crawlspace work)
• Snow or ice blocking the exterior vent cap in winter
• Improper installation: too many bends, wrong pipe diameter, or a vent that doesn't maintain upward slope

What to Do

Inspect the vent pipe from the top of the water heater to where it exits the building. Look for obvious blockages, separated sections, or sagging. Clear any debris from the exterior cap and make sure all sections are connected and angled upward.

Carbon monoxide warning: If anyone in the home has been experiencing headaches, dizziness, or nausea — especially near the water heater area — leave the house immediately and call 911. Install CO detectors near all gas appliances if you don't already have them.

Quick Diagnostic Checklist

Before calling a plumber, run through this list. It takes about 10 minutes and catches the majority of ignition failures:

1. Is the gas shutoff valve open? (Handle parallel to pipe = open)
2. Do other gas appliances in the house work?
3. What is the status light showing? Count the flashes.
4. Have you pressed the ECO reset button?
5. Do you store flammable liquids in the same room? (FVIR may have tripped)
6. Does the igniter produce a visible spark?
7. Have you replaced the igniter battery?
8. Is the pilot flame blue and steady — not yellow or flickering?
9. Is the thermocouple or thermopile clean and sitting in the pilot flame?
10. Is the flame arrestor screen clogged with lint or debris?
11. Is the exhaust vent pipe clear, connected, and sloping upward?

When to Call a Professional

Call a licensed plumber or HVAC technician if:

• You smell gas at any point during troubleshooting
• The gas control valve needs replacement
• The ECO reset button keeps tripping
• The status light shows 7 flashes or rapid continuous flashing
• You've worked through this entire checklist and the unit still won't light
• You suspect a CO problem (leave first, call 911, then call a plumber)
• There's water leaking from the tank — this usually means the tank itself has failed and repair isn't worth it

If your unit is showing signs it's near the end of its life (age, rust-colored water, rumbling noises, previous repairs), it may be time to weigh replacement. The average gas water heater lasts 8–12 years.

Typical Repair Costs

Here's what common repairs run so you can weigh repair vs. replacement:

• Thermocouple: $10–15 (DIY) or $100–150 (professional)
• Thermopile: $20–40 (DIY) or $100–175 (professional)
• Igniter: $15–35 (DIY) or $100–150 (professional)
• FVIR thermal fuse: $10–20 (DIY) or $75–125 (professional)
• Gas control valve: $150–250 (parts) + $100–175 (labor)
• Burner assembly: $50–120 (parts) + $100–150 (labor)
• Professional diagnostic visit: $75–150 (often waived if you proceed with the repair)

As a general rule: if the repair costs more than 50% of a new unit's price, or your water heater is over 10 years old, replacement is usually the better investment. See our full breakdown of water heater replacement costs in 2026.

How Old Is Your Water Heater?

Age is one of the most important factors in the repair-or-replace decision. A 4-year-old unit with a bad thermocouple is worth fixing. A 13-year-old unit with a failed gas valve is probably not.

Not sure when yours was made? The manufacture date is encoded in the serial number — but the format varies by brand. Use our free Water Heater Age Checker to find out instantly. Enter your brand and serial number and we'll decode it for you.