Common Tankless Water Heater Error Codes and When to Call for Repair

Ignition and flame failure codes

These appear when the unit tries to light and either fails to ignite or cannot confirm a flame. Common causes are a closed or partially closed gas valve, an interrupted gas supply, low gas pressure under load, a dirty flame sensor, a worn igniter, or air in the gas line after service elsewhere in the house.

Homeowner check: confirm the gas is on and that other gas appliances are working. If the furnace and range are fine and the heater still will not light, stop there — repeated ignition retries are not something to keep forcing.

Venting, air supply and exhaust codes

Tankless units monitor combustion airflow closely, and a code in this family means the unit is not getting the intake air or exhaust path it needs. Causes include a blocked intake or exhaust termination, ice or snow at the vent outlet in winter, nesting or debris, a disconnected or sagging vent section, or a blower fault.

Homeowner check: go outside and look at the vent terminations. Clearing snow drifted against them is reasonable and is a real winter cause here. Anything involving the vent piping itself is a call — this family of codes is the one most directly tied to combustion safety.

Overheat, scale and heat exchanger codes

These indicate the unit measured a temperature it should not have seen. Scale in the heat exchanger is a leading cause, because mineral buildup insulates the metal from the water and creates hot spots. Restricted flow from a clogged inlet filter, a partially closed valve or a failing circulation pump produces the same symptom.

This family is the strongest argument for annual descaling. Once these codes start appearing, the exchanger has usually been running hot for a while.

Descaling is the maintenance that resolves most of these before they turn into equipment failures — we cover it in our guide to Navien tankless maintenance and descaling in Chicagoland.

Flow sensor and water supply codes

Tankless units only fire when they detect flow. Codes here often mean the unit is not seeing the flow it expects: a clogged inlet screen, a stuck flow sensor, a partially closed service valve after a repair, low incoming water pressure, or a whole-house filter that is overdue for a cartridge change.

Homeowner check: verify that both isolation valves are fully open and that other fixtures have normal pressure. Changing a neglected whole-house filter cartridge is fair game. Cleaning the unit's inlet screen is a service task.

Gas pressure codes

A burner that fires hard needs adequate gas volume. Codes in this family show up in homes where the line feeding the unit is undersized, where several gas appliances run at once on a marginal supply, or where a regulator is failing. These are frequently installation-era problems that only reveal themselves on the coldest days, when the furnace and the water heater both want maximum input.

This is not a reset-and-hope situation. Gas supply issues get diagnosed with a manometer under load.

Condensate and freeze protection codes

Condensing tankless units produce acidic condensate that has to drain freely, usually through a neutralizer. A plugged trap, a frozen condensate line in an unheated space, or a spent neutralizer cartridge can all fault the unit. Freeze protection codes also appear when a unit installed in a garage or an unheated area lost power during a cold snap.

Homeowner check: look for standing water, a blocked drain line or a disconnected hose. Keep the unit powered through winter so its built-in freeze protection can operate.

When to reset and when to call

Resetting once is reasonable. A code can be triggered by a one-time event — a brief gas interruption, air in the line after unrelated work, a momentary power blip.

• Call if the same code returns after one reset
• Call immediately for any smell of gas, and leave the building first
• Call for anything involving the vent, exhaust or combustion air
• Call for repeated overheat or exchanger codes, which usually mean scale
• Call if the unit is under warranty, since documented professional service protects coverage
• Call if you have no hot water and the code will not clear

Is an error code a sign the unit needs replacing?

Usually not. Most codes come back to scale, venting, gas supply, ignition components or a sensor, and all of those are serviceable. Replacement enters the conversation when the heat exchanger itself has failed on an older unit, when parts are no longer available, or when repair costs have started stacking up on equipment that was undersized from the beginning.

We diagnose the fault first and give you both numbers — repair and replacement — so the decision is yours with real information behind it.

If your unit is showing a code you cannot clear, our tankless water heater repair and service page explains how we diagnose it and what to expect. You can also review our full range of plumbing work if the problem turns out to be elsewhere in the system.

Frequently asked questions