Takagi Error Code 391: Air-Fuel Ratio Rod Failure

What the code means

Takagi's service handbook for its condensing models (240, 340 and 540) says the heater displays code 391 if no flame is detected by the air-fuel ratio (AFR) rod. The AFR rod sits in the burner next to the flame sensor and igniter, and Takagi notes the green ground wire is part of the AFR and flame rod circuit, so a poor ground can trigger it. On the non-condensing 510 (T-D2) and 910 (T-M50) the display shows 391; the older 110 (T-KJr2) and 310 (T-K4) show it as two flashes on the board LED or code 39 on the remote. The condensing handbook also lists two flashes for 391. Don't confuse it with 111 (ignition failure, no flame after three tries) or 991 (abnormal combustion).

Likely causes

• The heater plugged into an outlet that isn't properly grounded (Takagi says a floating ground can cause this code)
• A loose, burned or cut wire on the AFR rod or flame rod circuit
• The green ground wire not installed properly or not making a good electrical connection
• The clear plastic tube between the proportional gas valve and the combustion chamber disconnected
• Oxidation or buildup on the AFR rod where it extends into the burner

Safe DIY checks

1. Check that the heater is plugged into a properly grounded outlet. If you're not sure the outlet is grounded, a plug-in outlet tester or an electrician can confirm it. Takagi lists this as the first step for code 391.
2. With the power off, remove the front cover and look at the wires on the front of the burner (the yellow AFR wire and orange flame sensor wire beside the sight glass) and the green ground wire. Make sure they're secure, not burned or cut, and that the ground wire is fastened and making contact.
3. Find the small clear plastic tube that runs from the proportional gas valve to the combustion chamber and make sure both ends are attached.
4. Clean the AFR rod. With the gas and power off, Takagi's procedure is to remove the rod assembly from the front of the burner, clean the part of the AFR rod and flame sensor that sits inside the burner with 100 grit sandpaper to a bright finish, and reassemble with all three wires connected. Be gentle: Takagi warns the rods bend out of place easily and must keep their original position. Don't clean the extensions on the outside, and check the gaskets and ceramic insulator for tears, chips or cracks.
5. Turn the gas and power back on and run hot water to check that the code is gone. If you aren't comfortable opening a gas appliance, the steps above are quick for a technician.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 101 Takagi Tankless Water Heater 101
• 11 Takagi Tankless Water Heater 11
• 111 Takagi Tankless Water Heater 111
• 121 Takagi Tankless Water Heater 121
• 291 Takagi Tankless Water Heater 291

All Takagi codes →

Sources

1. Takagi On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models (2000534342): Code 391, displayed if no flame is detected by the Air-Fuel Ratio (AFR) rod; steps: properly grounded socket, floating ground, wire connections and green ground wire (part of the AFR and flame rod circuit), clear plastic tube from proportional gas valve to combustion chamber, clean the AFR rod; error table lists two flashes; Cleaning the Rod Assembly procedure (100 grit sandpaper, yellow and orange wires, do not clean outside extensions, inspect ceramic insulator and gaskets)
2. Takagi Troubleshooting Guide 321923-000 for 110 (T-KJr2), 310 (T-K4) and 510 (T-D2): Error Code 391 (510) or two flashes / 39 (110, 310), Air-Fuel Ratio Rod Failure; check wire connections, clear plastic tube, clean the AFR; sensor cleaning to a bright finish, rods bend easily
3. Takagi Troubleshooting Guide 321925-000 for 910 (T-M50): Error Code 391, Air-Fuel Ratio Rod Failure, same three steps