GP51MP-24B-045-1 Hot Surface Ignitor replacement

1.5 is high but should still work as you are not getting a consistent error from that. You can cheat and temporarily bypass it by connecting the two leads together.

Everything was working. Now you see burner light but it goes immediately and then get the slow alternating flash. Anything else observed?

If not a loose connection to the FS then the board is suspect. It could be more sensitive than the old board was. Some bend the FS to change position. Make sure grounding connections are clean and tight. You need a good meter to measure the FS microamps but difficult if it is not lit. Try with the FS disconnected to see if same symptoms. Check again that ground to neutral is still close to zero ohms.

 

Hi Stuff,
Thanks. Here are the action items:
1. I will try the flame sensor disconnected. Not sure what to expect, but let's see.
2. I have ordered a new flame sensor just in case it makes the difference.
3. I will bypass the switch by connecting the two leads together. Will let you know.
4. I do not know if it was ever working well. Remember I moved into this home on Nov 2. At least the flame is ignited. The flame light goes away in 2-3 seconds.
5. Once I thought that the switch open indication came on before the watched set in. I will let you know if I notice any other symptom.
6. Keep in mind that when I moved in, I saw the alternating fast flashing. I disconnected the furnace a few times before it turned on. I could disconnect and restart a few times. Finally it just got stuck on fast flashing.

 

An HVAC specialist is coming in on Thursday. If not resolved till then, does not hurt to get a second opinion.

 

Hi Stuff,

Removing the flame sensor makes absolutely no difference. The furnace behaves identically to work and without the flame sensor

Could the flame sensor have gone bad?

 

I am now convinced that it is the flame sensor or the board. A new flame sensor is on the way. I will contact my seller to see if he can send me a replacement ASAP.

I have the HVAC guy coming in on Thursday. Hopefully things are resolved until then. Else, he might have an opinion. At this point, it might be nice to have someone else test these things too.

 

First, make sure when you are measuring ohms, that you do not have power on, and it usually also requires that you disconnect the item under test from the circuit.

The flame sensor actually generates a small dc voltage when it is heated. Well, many do, some change their resistance. You would need to know which type, and how to test it to say whether it was good or not.

Measuring voltage and having it mean something useful requires that you understand the reference points. For example, the low voltage acv control, interlock system (nominally 24vac, but 28vac or even higher may not be that unusual) may not read reliably if you measure it to ground. IOW, it may be floating, and to measure, you must be using one lead on the proper point, and that may not be ground.

Most burners controlled via a computer, try to start multiple times before they shut down. The exact number can vary. My boiler tries four times, then locks out. The manual usually describes the process that commonly is something like this:
- if a fan is involved, run the fan to purge out any unburned gasses, otherwise, pause some time to allow them to dispurse
- turn on the ignitor (assuming it is a hot surface one, a sparker may come on at the same time as the next step)
- turn on the gas valve
- determine if there is a flame detected
- if not, start over if not too many times where a fault has occurred.
- if too many faults, lock out the startup process

 

Hi Stuff,
Is it possible that I screwed something up while installing or checking?

 

IF the burner ignites, but then shuts off, then goes through that sequence again (after a delay to let unburned gasses escape), it's usually either the flame sensor itself, or the wiring that gets it to the control board, or the control board. You could ohm the leads of the wiring on that circuit to verify there isn't a bad wire or connector.

Depending on the type of flame sensor, you might be able to test it with a meter. If it changes resistance when it gets hot, there's usually a chart in the owner's manual or service manual (both might be available on-line if you don't have them) to indicate the normal range at various temperatures. If you were to take the thing out of the furnace, you could test it with a candle or a gas stove burner to see it change as it warms up. If the flame sensor generates an electrical signal as it gets heated, you could measure that if your meter can read down into the milliamp range. The manual would often say how to test it and what the values must be. If the wiring and the sensor are good, that really only leaves the control board.

But, there may be some other interlocks in the system that are not correct to continue to keep the burner on. Is this a closed combustion device (i.e, it draws inlet air from outside via a pipe)? If so, then once the fan turns on, if one of the pipes is partially blocked or otherwise compromised, there would be a pressure sensor that could shut the thing down.

 

Could have mis-aligned flame sensor or even the burner itself. As mentioned, could be failure of the replacement board.

 

Thank you Stuff and jadnashua.
I have no idea is this is a closed combustion chamber.
It could be a bad wiring. What should I expect when I ohm out the wiring? It should be 0, right?
It could be a misaligned/bad sensor or bad replacement board. I do not know as yet.

Let me ohm out the wiring today. There is a person coming out tomorrow to diagnose. Let him have a look at it too.
I will get back to you regarding what happens

 

Between the two leads of the wire there is an ohm of 0.3. With the low microAmp/milliAmp it is supposed to deliver is this OK?

 

If you touch the two meter probes together, does your meter read zero? Also, it may depend somewhat on what scale you're on and whether the meter is auto zeroing.

 

Diagnosed to a bad control board. Kicking myself right now. But it was a great learning experience.
Also, the burner cover was glued shut by the passing which had melted and glued things together.
Thanks so much Stuff and Jadnashua. You guys were right all along.

 

So everything working now?

So where did you get the board that failed?

 

$350 later, everything's working now.

I bought that board from HVACmegastore. They sell HVAC related stuff online. Supposedly they have their own website too.
What complicated matters was that they said, "we will take it back and give you 100% refund if a HVAC professionally done the board as a problem proven by a receipt". I suspect in the past people might have ordered and received stuff as troubleshooting strategy.

I think I just got unlucky. They sold me a bad board confusing the hell out of me. But you guys were right all along.

All in all a good learning experience. I need to o find a reliable stir to buy things from.