Work through it in this order
- Look at the display.Note any error code and look it up in the manual before doing anything else.
- Confirm power.Gas tankless units need electricity for controls and fans. Check that the unit is plugged in and the circuit breaker or GFCI outlet has not tripped.
- Confirm gas.Make sure the gas valve to the unit is open and that other gas appliances, such as the range, are working.
- Check whether it is house-wide.If every hot tap is cold, the problem is at the heater. If one fixture is cold, look at that fixture or its valve.
- Check flow.Open a hot tap fully. Very low flow may not trigger the unit; a clogged inlet filter can cause that.
- Stop and call.If those checks do not restore hot water, or a code keeps returning, call a licensed plumber rather than opening the unit.
What the symptoms suggest
| Symptom | Possible cause |
|---|---|
| Display is blank | No power: unplugged, tripped breaker or GFCI, or a control problem |
| Unit tries to fire, then shuts off | Gas supply, ignition or venting issue |
| Hot water at full flow, cold at low flow | Minimum activation flow not reached |
| Hot water weak and getting worse | Scale or a clogged inlet filter |
| Hot water fine except one fixture | Fixture, mixing valve or shower cartridge issue |
Things not to do
- Do not remove the front cover or touch the burner or gas valve inside
- Do not disconnect or adjust the vent
- Do not keep resetting through a repeating code
- Do not bypass a tripped GFCI outlet by plugging into an extension cord
An example morning
Illustrative example. Not a real job or customer.
A Fullerton household wakes to cold showers. The unit's display shows nothing.
- They check the outlet behind the unit and find the GFCI outlet has tripped.
- They press reset, and the display comes back on.
- Hot water returns, but they note the event in case it repeats.
- When it trips again two days later, they call a licensed plumber and an electrician to find out why.
A one-time trip can be harmless, but repeat trips point to a moisture or electrical issue that needs a professional.
Common questions
Does a gas tankless unit work during a power outage?
Most do not, because the controls and fan need electricity. Check your model's manual.
My range works but the heater will not fire. What next?
Gas reaches the house, so the issue is at the unit: its gas valve, ignition, venting or controls. Note the code and call a licensed plumber.
Could the problem be my water supply?
If cold water flow is also low across the house, check whether the main shutoff is fully open or whether the city has a service issue.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.