Tankless water heater flame sensor replacement

A flame sensor is a metal rod that sits in the burner flame. A tiny current passes through the flame, and that signal tells the control board the burner really lit. If the signal is weak, the heater shuts down for safety.

Figures are 2026 national estimates compiled from published cost guides such as HomeGuide, Angi and HomeAdvisor. Labor rates in high-cost metro areas often run above these ranges.

Typical costs

Codes that can point to the flame sensor

• Rinnai code 72 (flame sensing device), and ignition or flame failure codes such as 11 and 12
• Rheem code 13 (poor combustion detected by the flame rod)
• Navien E004 (false flame detection), where grounding is checked first

Clean or replace?

Carbon buildup weakens the signal. A technician may clean the rod, but manufacturers such as Rinnai often call for replacing it with a new gasket, especially if the signal stays low. Rinnai's guide calls for a flame signal of at least 1 microamp.

Why sensors keep fouling

A flame sensor that keeps getting dirty usually means the burn is not clean: restricted venting, a dirty burner, wrong gas pressure or the wrong altitude setting. Ask the technician to check combustion, not just swap the rod.

The rod sits inside the sealed combustion area, so this is not a homeowner job. See our tankless water heater repair cost guide for other repair prices.

Other causes of flame and ignition codes

Replacing the flame sensor fixes the code only if the sensor is the cause. A good technician checks the others first, because they are often cheaper.

Keep it clean longer

• Keep the area around the heater free of dust, lint and chemicals.
• Have venting inspected, especially after storms or pest activity.
• Stay on a yearly maintenance schedule, including combustion checks.

What to tell the technician

• The exact code and how often it appears
• Whether it happens at startup, during long showers, or only in cold or windy weather
• Whether anything changed recently: new gas appliances, a propane delivery, venting work or a storm
• When the unit was last descaled and serviced

These details help separate a worn flame rod from gas supply, venting or moisture problems, and often save a second visit. If the heater is older and has had several repairs, compare the quote with the tankless water heater installation cost for a new unit before approving major work.