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A.O. Smith Water Heater. What does error code of 991 indicate?

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Here you are Dorothy

AO Smith code 991 Solution for error code 991
Check the gas type of the water heater. Inspect the environment around the water heater. ... Check if there is grease and/or dirt in the burner (Part #101) and the fan motor (Part #103), especially if the water heater has been installed in a contaminated area.

Error code '991' for type ATI-110C-N by AO Smith. Cause and ...

More help here, but you probably need a service call, especially if it is a gas water heater.


Posted on Nov 30, 2020


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Multiple error codes can indicate failed PC board. Rheem will send PC board... so your computer-that-heats-water might have failed motherboard.
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991 error code on a Takagi T-K3 Flash gas tankless water heater

Re- set code!!!!
We have a Takagi TH3 DV 540 model.Air intake got blocked with snow, condensation etc.Once we cleared that then you just have to re-set it to clear the 991. On our model there are 4 tiny buttons across the bottom (Take off front cover) to find them. of the computer board assembly bottom left corner. Push the two buttons on the right simultaneously hold for 15-20 seconds or so, then turn the Takagi off then on again- should work now!
If you have a different model then those buttons will be in the bottom left corner still but running vertically- try doing as above with the bottom two buttons. We had to go without heat and hot water for 2 days until we could get hold of someone to tell us this. We called Seneca Supply 716-852-4744. Good luck

Apr 22, 2012 | RTO Takagi T-K3 Flash Gas Tankless Water...

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I have an A. O. Smith Model FPSH 40-261 hot water heater. It won't heat water. there is a green light on the control flashing. What does it mean?

AO Smith uses the Honeywell ICON gas valve with green indicator light.
Some models use White Rodgers Intellivent with a row of indicator lights.

The gas control blinks the error code:
Open following links to identify gas control thermostat and read possible error codes:

Jul 18, 2011 | Water Heaters

1 Answer

C4,76 what is it?

Before calling service technician, open following link for Rheen troubleshooting resources and possible error code help plus link to Rheem chat:

Typically, error codes are start-point for troubleshooting.
Error 76 is communication fault with remote control, which can be bad remote, bad wire, bad wiring connection, or bad part inside tankless.
C4 is oxygen sensor problem or bad venting.
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Jun 23, 2011 | Rheem Tankless Hot Water Heater...

1 Answer

Turn on power,15-20 seconds later,get error code 991, abnormal or imperfect combustion. natural gas at 7 on the water column, filter clean. other contributing factors for this code? can hi-limit switch...

First things... you have checked the inler filter andthe gas pressure, but have you checked and cleaned the thermistor(s)??? Also the flame sensor... clean that with a soft cloth...
Next turn down the temp with th dip switches that should tell you if it is the high limit...
Has the unit been cleaned with vinegar or CLR lately or ever??? there maybe buildup internally in the unit...
Is the flame a nice blue color or is it yellow???
Does it go out just before the unit cuts out???
Do the flames lift from the burner???

These are all things that I would be checking if I was coming to your home to check this unit...

Comment back with any other questions or comments...


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1 Answer

Code 991 continues to show.. will not clear... how do we clear that code???

to clear the 991 Takagi code, simultaneously press the bottom 2 grey buttons of the four just to the left of the flashing LED light. When the LED stops flashing, release and then press the reset button on the black panel about 8 inches above that

Jan 17, 2011 | RTO Takagi T-M1 Mobius Gas Tankless Water...

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Sometimes the hot water just goes cold

Here are a couple things to look for:

1) flashing burner light - a flashing burner light indicates the unit is getting an error code
common error codes:
A. continuous flashing - ignition failure or abnormal combustion; look for visible water in the machine indicating a leak, check gas pressure, debris or buildup on flame rod/igniter plug, or clean unit with compressed air
B. 3 flashes - error 16, abnormal outlet water temperature; typically this code indicates you'll need to descale you unit
C. 4 or 5 flashes - exotic error codes; you should contact Noritz technical support to deal with these codes

2. If there is no flashing code, try increasing the hot water flow. In the summer time you typically use less hot water; this can result in intermittent operation of your tankless heater. Increase water flow by either: A. cleaning your showerhead and removing scale deposits B. widening your flow restrictor C. Remove your flow restrictor in the showerhead

3.If none of these resolves your problem, you probably have debris in your flow sensor. Contact Noritz technical support for a walkthrough on removing debris from a flow sensor.

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