Hello,

Ran across the site and tried the listed Troubleshooting to determine if it's the flame sensor or the pressure switch port being clogged, etc.. None of those are my issue.

My issue is the furnace will start fine and run through a complete heat cycle and shut down. On the next call for heat the furnace will go through the normal start cycle except for when the flames achieve ignition it instantly shuts the inducer motor off. Then the lack of vaccumm pressure turns off the low pressure switch causing the 3 blinking lights to come on. I have double checked for continuity during the vacuum phase and the switch is functioning I have also jumped the wires after the initial startup on the low pressure switch and the inducer motor will still shut off at flame ignition but the gas valve stays on supplying the flames.

At some point after many many tries the furnace will eventually fire up. It could be anywhere from 20 minutes later to hours later.

I have checked and the pipes are not clogged for the vent or the intake. I have also checked and none of the pipes are clogged for the drains.

The issue resembles the videos I've seen on YouTube where people are experiencing a bad flame sensor however I have cleaned the old one and purchased a new one with the same end result - flame ignition and inducer power is turned off.

I've measured the voltage on the flame sensor and get between 1.4 - 1.6 mah (on the 200 vac setting - the lowest my multimeter has) when there is no flame and around 5.8-6.2 when there is flame present.

It is odd because at some point it will fire up 3 4 5 6 times in a row without issues and then randomly it will not work for hours.

I also bought a new primary limit switch thinking that perhaps it was still staying hot and the reason for the delayed start was the limit switch needing to cool off, it did not change anything.

I've tried tightening all the wire connections to no avail.

I tried removing the roll out sensors and taking them away from the flames when it's acting up thinking perhaps they were being tripped and it made no difference.

Another note worth mentioning perhaps is that after the first failed attempt to ignite when the inducer shuts off at the next cycle it kicks the blower motor on full speed and continues with the blower motor on until it finally reignites.

Any advice would be appreciated!

 

Oh, and I should add - it never goes into lock out... It just keeps trying over and over again.

 

If I remember right that Goodman starts on high fire. Check the low fire pressure switch for blockage.

 

Skaggsje

No block. You can physically hear the click of both switches engaging when the inducer runs. I've verified this with the multimeter and the continuity setting.

I have used a jumper wire to trick the system into seeing the switch as on. The system will fire up, immediately kill the inducer motor but because I still have it jumpered the gas valve stays on and the flames continue. However, because the inducer isn't running the flames creep back into the burner compartment. When I pull the jumper wire the valve immediately kills the gas and the flames. I still get the error code of 3 blinks even with it jumped.

Whatever is happening it's happening when the flame sensor is sending the signal spike back to the board (I theorize this because nothing else send a signal at ignition except the flame sensor right?). That signal is then killing the inducer motor.

A few thoughts are that perhaps there is a maximum time that the board allows for ignition and it's a split second off sometimes and kills the inducer a millisecond before the signal for a flame gets to the board.

Is there a way to test this? Is there a way to adjust it?

My other thought is that perhaps it's something somewhere shorting to ground? Not sure where to even check that?

I should add I bought the furnace add "new excess old inventory" at an auction and it's been sitting for a couple of years. So it's basically brand new inside, thus all the grounds look good (I've tightened them just to be sure).

Basically, I need to figure out why the power is killed to the inducer intermittently at ignition. So any since on how to trouble shoot that it's appreciated!

 

If you don't jump the pressure switch.... the inducer shuts off and the burner stops.... correct ?


It sounds to me like your low fire pressure switch is tripping in high fire. You may need to use a manometer on the pressure switch to see what's happening.

 

On most furnaces when ever the stat is calling for heat the inducer motor will be running. I assume you are jumpering the low fire pressure switch?

 

The inducer is supposed to run on a call for heat but on many furnaces.... the inducer will shut off in one minute if the pressure switch doesn't prove.

 

>



I would suppose that the inducer motor should continue to run IF the thermostat is calling for heat.

I'd verify that the thermostat IS calling for heat, which is quite likely.

The inducer motor is probably shutting off because either

1) the circuit board is bad and not keeping the 120 VAC to the inducer motor switched on

2) the inducer motor IS powered up with 120 VAC but is defective and not continuing to operate properly

Monitor the 120 VAC power to the inducer motor to identify which is the problem.

 

PJmax (not sure where I got pete from - sorry about that )

The inducer shuts off if it's jumpered or not (I.e. In both scenarios.) Even with it jumped it still gives the 3 blink code.

The gas only shuts off if I do not jump it. With it jumped the gas stays on, presumably because the system still sees the low fire as engaged (even though it blinks the 3 light code with it jumped).

How can I tell if it's tripping in high fire (I don't know what that even means honestly)?


Skaggsje,

Yes, jumped the low fire. For the hell of it I jumped the high fire to and the same result.

The inducer runs except for when the furnace is warm. In that instance it shuts off immediately at flame ignition. Something tells it to shut down.



It's random when it starts up. If it's been off for quite a while it starts up, sometimes it starts up fine 3, 4, 5, etc... Times in a row with no issue. The majority of the time though it enters this loop until something is just right and it fires up fine again. I thought maybe it was heat related but I changed the primary limit switch and no dice.

 

I also don't understand why the main blower comes on and stays on when it doesn't lite?

Could that be a sign of something?


Also, with the upper cabinet door on is it normal for the main cabinet door to push out about 1/4 inch at the center upon combustion and suck in so much that you can hear it pull air in from outside the cabinet when it shuts off? I don't think this is related because it doesn't lite if the door is on or off but it seems odd to me.

 

My name is Pete and it's in my tag line.

The burner door flexes on lighting or is it the blower door ?
It's normal for the blower door to get sucked in and make a noise although it's also an indication of undersized return ductwork.

The main blower is activated by time. It doesn't always know if the burner is lit or not.
Many furnaces will run the blower if a high limit stat opens. I don't think you have that problem. It would be a different trouble code.

I feel at this point you need a manometer to monitor the pressure switch levels.

This is a fairly new furnace..... have you checked with the installer yet ?

 

>


It shouldn't do that. So it's shutting off because of a defect ---- probably a bad circuit board or perhaps a defect with the inducer motor or fan.

You may have more than one problem. If so, you should fix them one at a time.

Personally, I'd start with determining why the inducer motor is shutting off when it shouldn't.

 

Hi Pete,

I purchased the furnace at an auction of surplus new old stock furnaces, thus - I am the installer.

It's the burner compartment that flexes at ignition. It puffs outward at ignition and sucks in slightly when the flames are extinguished. I don't think it has anything to do with the problem though because as I mentioned, it has the same issue with or without the door installed.

I have a digital manometer being delivered tomorrow.

Where should I start? I assume at the low pressure switch but what am i looking for specifically?

Btw, sorry about delayed responses during the week. Long hours = weekend warrior.

 

A thought, is it possible that at ignition it's creating a drop in pressure that (for a split second) breaks the vacuum and trips the switch off?

 

I've never seen that before. That doesn't sound good.
Do you have a fresh air in vent line to your furnace ?

Anything that disturbs the flue pressure will cause an upset.

 

https://youtu.be/7HuY4ECrXxo

Took a video of it last weekend.

Yes, direct vented. It was going up to one of those combined vents but it was dripping water out of the intake. So this past weekend I cut it off the combined and snorkled it on its own line away from the combustion discharge.

This video was before I cut it. It does the same thing after though and as you can see even with the door off it makes no difference.

 

Good video.

After the burner shuts off...... the system MUST go thru a new prepurge cycle. Your system is trying to relight almost immediately. That is not correct! I'd say that's a bad control board.

Regardless of any outside influence..... when ignition is interrupted..... the board must start the timing cycle over.

Since the burner is shutting off approx four seconds after lighting.... that points directly to a flame sense issue.

 

Hi Pete,

Thanks for the reply!

Maybe a narrative would help.

I have the service manual and have checked the startup sequence - it is correct.

What's actually happening is the flame goes out and the system does go back through its prepurge cycle. The normal startup procedure is that it flips on the ignitor for a few seconds (I suspect to see if it's working before it tries to light), then the ignitor goes off and it begins the prepurge gas cycle at which point it flips the ignitor back on again, then approximately 7 seconds later it ignites the flames. Immediately at ignition the inducer shuts down. However, because the inducer is still spinning down the system has sufficient vacuum to keep the switches engaged and thus the flames stay on because there is vacuum and the system is sensing flames (verified via multimeter). As soon as the inducer spins down far enough you can physically hear the switches disengage and that is when the flames immediately go out and the system sends the error code of 3 blinks (because the low pressure switch is the first to click off).

I've replaced the flame sensor with an entirely new one and got the same before and after ignition readings on the multimeter, and of course, the same results.

As previously mentioned, if I jump the low pressure switch at startup the flames will actually stay on even after the inducer fully spins down. So the system is seeing the flame sensor reading so that leads me to believe the flame sensor is fine. This also tells me that it seems to definitely be linked to the Low Pressure switch signal and not any other area on the board.

My thoughts are this is all caused by the inducer shutting off and the vacuum dropping to the low pressure switch.

The only problem is I can't figure out why the inducer is shutting down at ignition and how to fix that. The only other logical thing I can think of is that perhaps at ignition that poof of air pressure is momentarily interrupting the pressure to the switch just enough to cause the contact to break inside the switch and kill the inducer but it regains contact so quickly again that it keeps the flames running until the inducer spins all the way down.

How can I check the pressure during ignition and what should I be checking for?

Is there any other way to check what is happening with the electric on the inducer?

Also, could it perhaps be that there is to much gas being injected at ignition causing to big of a combustion and creating the pressure change? How could I check that with the manometer?

Sorry for so many questions!

 

If you have a manometer it needs to be Tee'd into the line so that you can watch what the vacuum does. A meter connected to the pressure switch will allow you to know when the switch opens.