Gas tankless water heaters show an error code when they can't light or can't keep the flame lit. The code number depends on the brand (check your owner's manual or the label inside the cover), but the causes are usually the same.
Common causes
- Gas supply problems: a gas valve not fully open, or another gas appliance running out of gas supply at the same time
- Undersized gas line: tankless units need a lot of gas. A line sized for an old tank heater may not keep up.
- Venting or air intake problems: a blocked vent, bird nest, or debris in the intake
- Dirty flame sensor or igniter
- Scale buildup affecting operation, especially if the unit hasn't been flushed
Safe checks you can do
- Make sure the gas valve to the unit is fully open.
- Check that other gas appliances are working.
- Look at the outside vent and intake for blockages.
- Turn the unit off and back on to reset it.
If the error comes back, stop there. Anything involving the gas line, burner, or internal parts should be handled by a licensed plumber.
Prevent it
Most tankless manufacturers recommend regular flushing to remove scale. In areas with hard water, this matters even more.