I just had a gas valve with a shorted coil do that.

 

Though I've never seen this failure, I did half-wonder last night if a bad gas valve could do this; open just long enough to light but not be strong enough to stay open, though you would think holding open was easier than opening. I should be able to test the coil. Good idea.

Reversed polarity on the line voltage can do funny things ...

Very interesting idea. Something I'll definitely rule out easily enough.

 

Reversed polarity on the line voltage can do funny things and I've found reverse voltage on a furnace that had been installed for 20 years and all of a sudden the polarity is reversed. I guess somebody must have done some electrical work over the summer and messed it up.

 

I would check the ground or better yet, run a ground wire from the flame sensor to the ground strip. Also burner condition can play an issue, rust etc. What's the reading on the flame sensor (if it runs long enough for that)?

Edit: I just re-read your post about checking the grounds. I would check the burner face for rust and make sure the burner rack is grounded.

 

Whatever you end up finding, just make sure you move that sensor back to burner #6. If 6 fails to ignite, but 1-5 light just fine...

 

I am my wit's end on this one, fellas. Unit fires up as normal, but gas valve shuts off after 1-2 seconds, exactly as if the flame isn't sensed.

I'm the second guy in there, which of course we all hate. Homeowner says the other guy diagnosed it as a bad board but he couldn't afford it then, so wanted me to replace it. I could tell the other guy even tried a new flame sensor and for some reason, the flame sensor wire is spliced to the harness plug, probably because he also replaced as much of it as he could. It was a good splice, and I also tried another wire with a good splice. I also put some copper strands as shims inside the harness plug to ensure good contact to the board pin. So I think it must be the board, too. I get a used, working, identical part number board from a seller on eBay that I've gotten many good boards from.

I install it yesterday and get the exact same symptom. So it's probably not the board. I then tried another flame sensor. I move the flame sensor from burner #6 to burner #5 so it gets hit with fire just a little bit sooner. I bend the sensor to put even more of it into the flame. I get a good continuity on ground between the chassis of the flame sensor and all other grounds on the machine that I can think of.

If I let the machine go to lockout, I just get fault code 1 for ignition lockout. I do not get 7 flashes which would be low flame sense.

View attachment 840813 View attachment 840815 View attachment 840817

Please explain the bolded.

 

Simple beep continuity test. I will take a different meter with me next time that can do a microamps current test. I was a bit surprised my Fieldpiece that I've been using for years didn't have the option, and no way is the clamp gonna do it.

 

What happens if you jump the p-switch?

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Had that problem on a Carrier today.
Code #34
No flame sensed
Lights maybe 1 second
I waited till the inducer got up to speed then jumped and it 🔥 right up.

 

What happens if you jump the p-switch?

Ah, I did think of that one. I did jump the p-switch as soon as induction motor started, to make sure that stayed closed. No change in symptom.

 

Sorry, I should have been more specific.

What is the chassis of the flame sensor you mentioned in post 1?

In general you don’t want continuity from the rod on the sensor to ground but you do want a good path from the burner face to ground on the board.

You may also want to check voltage to the sensor to ground.

One more thought. Generally trial for ignition lasts 4 seconds, so from the time the gas valve is energized till the time it is de-energized is 4 seconds, but you say it is about 2 seconds which is the same only half. Try disconnecting the sensor wire and see if the valve stays energized the full 4 seconds. If it does then it would be a sensor or wire problem, and f not you would be looking at board or board inputs being off.

 

Sorry, I should have been more specific.

What is the chassis of the flame sensor you mentioned in post 1?

In general you don’t want continuity from the rod on the sensor to ground but you do want a good path from the burner face to ground on the board.

You may also want to check voltage to the sensor to ground.

Ah, I see. I was checking continuity from the screw that attaches the flame sensor to the side of the burner area. I didn't even think that it's where the flame exits the burner that might be the part that matters.

One more thought. Generally trial for ignition lasts 4 seconds, so from the time the gas valve is energized till the time it is de-energized is 4 seconds, but you say it is about 2 seconds which is the same only half. Try disconnecting the sensor wire and see if the valve stays energized the full 4 seconds. If it does then it would be a sensor or wire problem, and f not you would be looking at board or board inputs being off.

This is a great idea. I should have timed it more precisely, and it SEEMED like sometimes it might have been longer than other times.

 

Dust/lint buildup on the backside of the control board can cause flame sense issues. I had that one whip my butt for a bit once upon a time.
Poor wiring/ground on the line voltage coming to the furnace as well can do that. For example if the furnace is on a switch or even recept with poor connections can cause issues and cause ya to throw parts at one because everything At the furnace appeared to be good.
Could temporally for testing, jumper 24v to the valve when it calls for the valve and see if it stays burning to eliminate problem with the valve itself.

 

Dust/lint buildup on the backside of the control board can cause flame sense issues. I had that one whip my butt for a bit once upon a time.
Poor wiring/ground on the line voltage coming to the furnace as well can do that. For example if the furnace is on a switch or even recept with poor connections can cause issues and cause ya to throw parts at one because everything At the furnace appeared to be good.

I've had many quit because of those loose connections, usually from a backstabbed wire into a cheap switch. And this one is on a switch, so I will check that.

Could temporally for testing, jumper 24v to the valve when it calls for the valve and see if it stays burning to eliminate problem with the valve itself.

Another great idea. I will override the flame sensor and keep the gas valve energized with a jumper. If it still goes off, it's the valve!

 

@ MAC702

You have been around since 2019, plenty of posts, why not apply real quick for PRO status? I invite you to, my friend. There is so much more on this subject in that area. Appears you do this for a living, might just be streamed line straight in to PRO based on your past posts?

If you were PRO, this thread could be moved over to the PRO area and we could post service manuals, parts lists, and start digging deeper into things.

Though you are being assisted here in the tech to tech, still we are all skating on thin ice with DIY assistance.

Here is that link for PRO, I am sure our fellow member here, Poodle Head, will be more than happy to fast track you through if he feels you are legit, which it appears you may be?

https://hvac-talk.com/vbb/threads/2235857

 

i have had a dirty inducer wheel cause what appeared to be poor flame rectification, it would drop out the pressure switch. it was a lennox tho....
sounds like you have dealt with the pressure switch issue.
i would also suggest checking all the ground wires..

 

I had a no heat call last week, found tripped on ignition fault. Carrier gas pack 3-4 ton unit. Take the panel off, and it's like, uh oh. This isn't good.

Before I even saw the flashy code, much less even counted the flashes, my first thought was: This is call back city. Slow down, take your time. Don't want my name added to the list of techs before me.

What did I see? New parts. A bunch of them. That's never good. As 600 might say, someone shot the parts cannon at this baby. Now I see the flashy code, count the flashes, reset, and of course, it fires off. Do that a couple of more times, fires off. Now I have to slow down even more, you know, just sit back and have a chat with the unit. Bad ground did cross my mind, but for some reason, I suspected something else, maybe because everything did look really good and clean. For a 20 year old unit anyway.

So I'm just sitting back, looking around. Maybe a wind thing? Probably not, quite a few units on the roof, seems like if it were the wind other units would be having problems and someone would have picked up on that through the years. Gas pressure? Probably not, several gas meters for various tenants, no new units added to that gas main.

I fire it a couple of more times, and I'm thinking, it's right here. Fire off a couple of more times, and then realize, spark doesn't sound real healthy. "Sounds" like a weak spark; can't really see the spark. A couple more fire offs, gotta be a weak spark. For whatever reason I pull the the spark wire off of the coil, what the heck? No wire? Just a hole where the wire should be!

I end up trimming off an inch of insulation till I get good wire. Stick it back on the coil, cycle heater, and now I have a loud crisp clean spark. Bada Boom Bada Bing (which is what made me remember that unit just now, LOL).

Let it run while I'm cleaning up and doing paperwork. I must have a fetish for touching motors or something. Put my hand on the induced draft motor and it's hot! Cap is okay. Measure the amps and it is three times what it is rated for, voltage okay. So I write up the motor, and call it a job well done.

Somebody stuck the spark wire on the coil without ever looking to see if there was actually a wire there!

Oh well, no physical strain on this call, LOL.

 

I’ve had spider webs built up in the burner manifold cause it, behind the orifaces. Usually on RTU’s.


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Resolution, I guess. Supposedly, another tech with another board and another flame sensor fixed it. I never got around to chatting with him about it. I'm dubious, but all I know is that I got off the hook gently. Sorry I didn't update you fellas earlier.

 

Still a reminder to get your PRO status, so much easier for us to assist folks within the PRO area.

 

Had them internally drop a call for heat before. Either bad soldier joint on w, or dropping somewhere else.
Usually shows up as unit fires up, and then shuts down, restarts. No code.

Or a nest thermostat. Had that one 3 weeks ago.