Code: 441🚿 Takagi Tankless Water Heater

Takagi Error Code 441: Flow Sensor Failure in a Linked System

Reviewed 2026-10-24 · Based on 2 manufacturer & service sources

What the code means

Takagi's service handbook for the condensing 240, 340 and 540 models lists code 441 as "Flow Sensor Failure (Easy-Link or Multi-Link System Only)," shown with two flashes of the computer board LED. Takagi's troubleshooting guide for the older 110, 310 and 510 models lists it for the 510 (T-D2) only, as part of an Easy-Link system: the master unit signaled an additional heater to turn on, and that heater didn't respond. The flow sensor uses a small impeller that spins as water runs through it, and the heater won't fire unless it sees that flow. A single heater that isn't linked to others won't show 441; if a standalone heater simply doesn't start when you open a tap, that's a different problem.

Likely causes

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

  1. Check which heater in the group is showing 441. Takagi's handbook says to test that heater on its own, disconnected from the linked system, so a technician may need to do this step on a commercial setup.
  2. Make sure the water supply valves to that heater are fully open.
  3. If the heater has a controller, push the INFO button until the arrow points to "Flow" and open a hot tap. Takagi says a flow rate should show during operation; if it doesn't, keep going.
  4. Clean the inlet water filter: turn off the gas, power and cold water to the heater, open hot taps to relieve pressure, then unscrew the cold inlet filter over a bucket, brush off any debris, and screw it back in hand-tight only.
  5. If the code comes back after the filter is clean, the next steps mean opening the cabinet, so call a technician unless you're comfortable working on gas appliances.
  6. If your water is hard, descale the heater on the schedule in your manual, since scale can collect in the flow sensor.

🛠 When to call a pro

  • Have a technician check the flow sensor/control valve wiring for broken or disconnected wires (Figure 41 or 42 in Takagi's service handbook).
  • With gas and power off, the technician removes the flow sensor and blows into its inlet. Takagi says the impeller should spin freely for a few seconds; if it stops abruptly or doesn't spin, clean or replace the sensor. Takagi warns not to take the sensor apart.
  • If the linked system still won't bring the heater online, Takagi's handbook also lists 761 for communication errors in an Easy-Link or Multi-Link system, so the technician should check the linking setup too.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Tankless water heater flush kit

Descaling helps keep scale out of the flow sensor in hard-water areas.

Search eBay →
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Multimeter

For a technician checking the flow sensor wiring.

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⚙

Carbon monoxide alarm

Recommended for any home with a gas water heater.

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Sources

  1. Takagi On-Demand Water Heater Service Handbook for 240, 340 and 540 Condensing Models (2000534342): error code table, 441 Flow Sensor Failure (Easy-Link or Multi-Link System), two flashes; Code 441 steps: disconnect from the linked system and test as an individual unit, ensure the water supply is on, check flow sensor/control valve wires (Figure 41 p. 49 or Figure 42 p. 50), verify flow rate with the controller INFO button, check the inline water filter for debris, remove the flow sensor/control valve (p. 51) and check it per Checking the Flow Sensor (p. 36: impeller should spin freely when you blow into the inlet; do not disassemble), clean or replace as necessary; 761 Communication Error in Easy-Link or Multi-Link System
  2. Takagi T-KJr2, T-K4, T-D2 Troubleshooting Guide (321923): 441 Flow Sensor (Easy-Link System only), no code on 110/310; Error Code 441 [510 (T-D2)]: master unit signaled an additional heater to activate and it did not; verify water supply, wires connected to the computer board, inlet filter debris, flow sensor debris; Cleaning the Filter procedure

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.