An expert guide to diagnosing and repairing the most common gas water heater problems.

There are three common issues with a gas water heater: 1) It doesn't heat water at all, 2) it doesn't heat enough water, or 3) its pilot light does not stay lit. For problems that apply to all types of water heaters (both gas and electric), see Water Heater Problems & Repairs.

Water Heater Does Not Heat Water 

If a gas water heater fails to heat water, perform these diagnostics:

1) Be sure the gas is turned on. Turn the gas control knob to PILOT to prevent the burner from igniting when you are looking inside. Remove the metal cover at the bottom of the water heater and look to see if the burner and/or pilot light—the small flame at the end of the pilot gas supply tube—are lit.

2) If the pilot light has gone out, follow the instructions on the tank to relight it. It’s also possible that the gas inlet valve has been closed partially or all the way. If so, turn the handle parallel to the line and relight the pilot. If the pilot won’t light, the thermocouple may be defective—either call your gas utility company (a free service in many areas) or an appliance repairperson. (Newer water heaters may have a glow plug or spark ignitor instead of a pilot—follow the instructions in your owner’s manual.) 

3) If the burner is not on, replace the cover and make sure the thermostat is set to about 120 degrees F. If it isn’t, adjust it, turn on a hot water faucet, and then wait a few minutes to see if the water heater burner ignites. If it doesn’t ignite, leave the hot water running and try lowering and then raising the temperature setting on the dial until the burner ignites.

If the burner ignites, replace the cover and turn the thermostat back to an appropriate setting. If the burner doesn’t ignite, have the water heater checked out by an appliance repairperson. It could be that the heater’s thermostat is defective. Replacement is best left to an appliance repairperson or a water heater specialist.

4) Smell for gas. If you smell a garliclike scent, turn the gas valve control to Off (you may have to push down to turn it). Wait until the gas smell has dissipated before relighting the pilot light. If the gas smell is strong and doesn’t dissipate, immediately turn off the gas supply valve, ventilate the space, and call your utility company from a remote location.

Water Does Not Get Hot Enough

When a gas water heater doesn’t provide enough hot water, do the following:

1. Turn the thermostat dial to a hotter setting. If the flame ignites when you do this, leave the dial at that setting and then check the temperature of the water in about an hour. To do this, put a meat thermometer into a cup and let hot water from a faucet flow into the cup for about a minute. Then read the temperature.

2. If you don't hear the water heater's burner ignite and/or the water just keeps getting colder, check the pilot light to be sure it hasn’t gone out. If it has, follow the manufacturer’s instructions on how to relight it. Relighting a water heater usually involves turning the water heater's gas valve to Off and then to Pilot. You typically hold down a button, light the pilot, and wait until a status light signals that you can turn the dial from Pilot to On.

Pilot Light Does Not Stay Lit

If your gas water heater's pilot light goes out, see How to Relight a Gas Water Heater. If the pilot light on your gas water heater keeps going out, your pilot light’s orifice could be dirty and clogged, making the flame small and unsustainable. Check to make sure there is no obvious dirt and debris clogging it.

If this is not the issue, there may be a problem with the thermocouple. This is the part that extends from the temperature control into the flame of a pilot light and helps the main burner fire up once the water temperature has gone too low. The pilot light could be affected if the thermocouple is faulty or does not come into contact with the pilot flame. If you are handy, you can replace this part yourself. If not, call in a service technician.

Low gas pressure or a malfunctioning gas control valve could also cause a pilot light to go out repeatedly. Always call an appliance repairperson or your gas company if you suspect this is the case.