Typical/Expected Fuel air current measurement

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Conde

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I have a Takagi T-H3J-DV water heater in a domestic radiant heating application. I am getting the error code 391 (Air-fuel ratio rod failure). I have cleaned and replaced the sensor multiple times but the issue seems to come back intermittently. I did put the heater in diagnostic mode and was able to read the current value on the air-fuel rod. I was measuring 7 µA on a brand new sensor with the unit operating is steady-state conditions.

My main question is, what would be an expected current value? The range on the diagnostic mode is 0-999 µA but that does not tell us much. I have reached out to Takagi to as what value/level of current would set error code 391 but I am not hopeful for a reply. I would assume the fault has some level+time+hysteresis type value.


Thanks, I love this forum for the helpful answers!
 

John Gayewski

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These are junk heaters. I was working on one with this same issue not long ago. There is a number you can call and the tech was somewhat helpful. You need to measure the gas to make sure the pressure drop is within spec. I haven't heard anything about the one I was working on, but I had ordered a new set of gaskets to take the burner box apart and clean it if it continued its issues.

My recommendation was to get a different heater as it had never been cleaned and it was 10 or more years old.

How old is this heater?
 

Conde

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This unit is 8 years old. I did take the combustion chamber apart this summer and cleaned it pretty thoroughly. I will try the Takagi service number as well.
 

Conde

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I called the Takagi service number and it comes up as AO Smith which apparently owns Takagi and just about every brand as well. They were somewhat helpful but not detailed. I wanted to know the Air-fuel limit that sets code 391 but he did not know. Just said that short cycling and long duration firing can lead to soot buildup so.... Super helpful.
 

Fitter30

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