Read the display first
Most tankless units show an error code when something is wrong. The code is the quickest clue, and the manual explains what each one means. Write the code down before you reset anything, because a plumber uses it.
Codes differ by brand, but they fall into a few families: ignition, flow, temperature, venting and sensor faults.
Scale on the heat exchanger
Minerals in the water coat the heat exchanger and narrow the passage. The unit overheats, trips a limit or delivers lukewarm water. A flush with a descaling solution cures it, and it is the most common fix for an older unit that has never been serviced.
See tankless water heater service for what a flush involves.
Gas, power and ignition
No ignition
Check the gas valve, the supply and the igniter.
Electric unit trips the breaker
A failing element or a shorted wire.
Display dead
Check power, fuses and the control board.
Burner lights then drops
A dirty flame sensor or weak gas supply.
Local note: water quality and winter in Marblehead
Marblehead buys all of its water from the MWRA. The town's 2021 water letter reports lead and copper results below the action levels but does not give a hardness figure, so test your tap or ask the Water and Sewer Department if you want to know how fast scale will build in your unit.
Coastal winters bring cold snaps that freeze condensate lines and exterior vents. If your unit sits in an unheated garage, basement corner or porch, have the plumber check the freeze protection before January.
Error code meanings and service intervals come from the manufacturer's guidelines for your model, so keep the manual and the install record together.
Tankless gone cold?
Call the 24/7 line and a plumber can read the code.
Venting and condensate
A blocked or damaged vent trips a safety switch. A condensing unit also drains acidic condensate through a small tube, and if that tube clogs or freezes in a cold space, the unit shuts down. Clearing the condensate line is a common winter fix.
A sooty vent or a smell of gas is a safety issue. Shut the unit off and call.
Flow too low to fire
Tankless units need a minimum flow to switch on. A clogged inlet screen, a partly closed valve or a faucet aerator full of scale can drop the flow below the threshold. Clean the inlet filter and the aerators first.
The cold water sandwich, a short burst of cold between hot draws, is a normal quirk, not a failure.
When to call
If the code returns after a reset, the cause is still present, and running the unit that way can damage it. A plumber reads the code, checks the supply and vent, and flushes the exchanger. See water heater repair if you also have a tank-type unit in the house.
A short checklist before you call
Check that the unit has power, that the gas valve or the breaker is on and that the isolation valves are open. Look at the inlet filter for debris, and run a different hot tap to see if the problem is one fixture.
Note the error code, the time it began and whether it follows a pattern, such as only at low flow or only in the morning. A plumber uses those clues to go straight to the cause.
Preventing the next shutdown
Flush the unit yearly, clean the inlet filter, keep the vent clear and protect the condensate line from freezing. Keep the area around the unit free of storage and dust.
If the water supply carries a lot of scale, a scale inhibitor or softener ahead of the heater can slow the buildup. Ask a plumber what suits your water.
Safety first with gas and venting faults
If you smell gas, see soot on the unit or hear a rumble from the burner, shut the unit off and leave the area before you call anyone. Gas and combustion faults are not do-it-yourself repairs, and a working carbon monoxide detector near the heater is essential.
A plumber checks the gas pressure, the burner flame, the heat exchanger and the vent before putting the unit back in service. Ask for the combustion readings and keep them with your records.
If the heater is an electric model, the same rule applies to the breaker. A breaker that trips again after a reset means a fault, and holding it on can damage the wiring.
Keep the manual, the install date and every service receipt in one folder. When a code appears, the history shows whether it is new or a repeat, and a repeat points straight at the cause. That record also supports a warranty claim if the manufacturer asks for proof of yearly flushing.
Tankless troubleshooting questions
It has detected a fault such as ignition, flow, temperature or venting. The manual and the code history tell the plumber where to look.
A plumber connects a pump and circulates a descaling solution, then rinses. Yearly is typical.
Yes, on condensing models. Clearing and protecting the line fixes it.
Scale, low flow or a failing sensor can cause it.
No. A code that returns means the cause is still present.
Yes. See Swampscott and Salem.
At each flush, and any time flow seems low.
Yes. Most need electricity for the fan and controls, even on gas.