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How to reset an error code 99 on a Takagi T-K4-IN Tankless Water Heater

Symptoms: Beeping 5x with error code 99
This is a 991 - hard lockout due to the heater sensing that it is burning “dirty”.
This burning dirty state might be a false stuck state due to high winds which momentarily blew crud into the unit. Or it might require a technician to come by to clean the remove and clean the burner unit.

Steps to resolve error code 99.

Use a Phillips screwdriver to remove the front plate of the unit.
Locate the control board on the lower left of the unit that is obscured by a translucent plastic. ​
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Remove the plastic cover or flip up the cover so that you can see the control board underneath. ​
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Locate the four special purpose buttons along the bottom edge of the board. Some models have the buttons vertical and some horizontal. Our model has the buttons horizontal. ​
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Locate the red status LED which should be blinking.

6. Press and hold down the left most two buttons until the red LED light becomes a solid red. Then release the two buttons. ​
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7. Locate the power toggle above the board we were working on. Toggle it off then on to reset the unit. ​
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8. Check the console to set an appropriate temperature. Takagi support recommends 120 F, we set ours to 122 F.
9. Go to a faucet to run the hot water.
10. Look for signs that the tankless is operating. It should the sound like a condenser fan.
11. If the hot water is working, then great the problem is solved or at least deferred till another day. If the hot water is not working then you need to find a tech in your area who can remove and clean the burner unit.
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