Carrier Furnace Ignition Lockout - why?

Hi all,
Got a furnace problem that has me running in circles. Perhaps some quick advice can point me in the right direction at least. We have a Carrier furnace, older that 5 years old, and this year is has been failing to ignite, flashing error code 14 (ignition lockout) causing it to wait 3 hours before the next attempt. FYI here is the code legend:
#14 default= ignition lockout, control will reset after 3 hours. Refer to #34.
#34 default= ignition proving fault. Check for gas valve turned off, oxide build up on flame sensor, main shut valve turned off, Green wire must be connected to furnace sheet metal, proper flame sense microamps (.5 micro amps DC minimum), inadequit flame carry over or rough ignition, low inlet gas pressure.
Now, a little history: 1) Four years ago we had a crawlspace dug out and the project and the project went poorly, causing the basement to be exposed to excessive dust and moisture for much of that winter. 2) This winter we began having trouble with the thermostat. The backlight would not light, it would display gibberish, and sometimes not display anything. Shortly thereafter we noticed that this behavior happened at the same time the furnace had the ignition lockout. 3) We called an independent contractor and described the problems. The tech wasn't aware or (or wasn't concerned with) the codes and was only concerned with the thermostat. He indeed found and straightened some bent pins and we could notice a tighter fit when we closed the cover of the stat. He figured that solved the problem, and he never even looked at the furnace itself. 4) A month later the lockout recurred, and the contractor sent another guy on the callback. The furnace had restarted by then so he didn't see the codes, and I don't think we mentioned them when we made the emergency call. Anyway, he was certain it was an overheating issue and when he went in he found the coil was *very* dirty (from the crawlspace project) and had to scrape it with cleaning solution. 5) Now, another month later, the problem has returned.
I don't know which direction to take this. I don't want to keep bring techs out who don't fix the root cause (whatever it is). Before I invest in another call, perhaps someone could tell me: - Could this problem really be caused by a defective stat or dirty coil? The stat HAS been loose and the coil HAS been dirty, but the error codes don't mention these, and don't seem to allude to them. For that matter, if the ignition is locked out, will it cause the stat to act funky when I use it? I don't want to pay for another scraping, or a new coil, if I don't have to. - If I call the techs back, after I make sure they are aware of the codes, what directions should I give them? - Is there anything I can check on my own fairly easily before resorting to another callback?
Thanks for any advice, -Larry
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snipped-for-privacy@gmail.com wrote:

Larry;
1. The furnace has been subjected to abuse. [Hence the dirty coil causes the heating unit to 'overheat.'] Change your filter more often, tighten up any return air duct leakage, and that wouldn't have happened. Have more annual inspections [as opposed to having a tech only look when there "is" a problem in the future.]
2. Your ignition lock out is from flame proving failure. Likely the flame sensor has become fouled and needs a replacement [like a $15 part or less.]
3. It's likely from reading your story that you will experience other problems as well. It is *unlikely* you have a failed firebox [heat exchanger] since the fusible link [provided by the mfg.] hasn't failed or there hasn't been a flame rollout event 'yet.' [Depending on model.] But because of the abuse from the past, it's likely you have a clogged indoor blower motor and clogged blower as well, [evident since the evaporator coil was clogged.] So look forward to a indoor motor replacement soon.
4. It's likely you have some supply / return duct repair ahead as well, if there was some crawl space work, generally speaking, other trades tend to not respect ductwork like you'd think they should.
--
Zyp



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Thanks for the advice Zyp. Indeed I make no excuses for the neglect the furnace has had, and accept the consequences. I will replace the sensor immediately and have an inspection done since the cooling season is not far away. One last question: why does my thermostat acting the way it does? When there is a lockout, does the furnace send a corrupt control signal to it and cause its display to go haywire or is the thermostat poor and cannot interpret what the furnace is trying to tell it?
thanks again, Larry
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snipped-for-privacy@gmail.com wrote:

If you interupt the 24 volt to the thermostat [Totalline / Carrier] the display will die.
--
Zyp



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Years ago, I was assistant to the assistant, on a job where we were extending some duct. Family was building addition to the house, and wanted to tie that into the HVAC. We needed to run a rectangular duct through a cinder block foundation wall. Spent the entire day with a SDS hammer drill, cutting out cinder blocks while standing on a ladder.
The next day, we came back. The cable TV guy had been there, and had run a cable TV wire diagonally through the hole we spent all day making.
So, Zyp, you are correct about that.
--
Christopher A. Young
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On Sat, 22 Mar 2008 09:33:32 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

Perhaps you should run in "squares" instead of "circles"? After all, you pay for work repeatedly not done correctly so Id figure you'll probably run in "squares" just fine. Bubba
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Don Ocean posted for all of us...

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Tekkie Don\'t bother to thank me, I do this as a public service.

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On Mar 22, 12:33 pm, snipped-for-privacy@gmail.com wrote:

These could have contributed to the problem or been the only problems at the time. Certainly they needed to be addressed. As far as your lock out goes it could be any number of problems related to gas pressure, dirty sensor(flame), ignition control failure these will only be able to be found by a properly trained tech. Unless you have some knowledge of HVAC controls you don't want to mess with gas. The next tech that comes out should be made aware of all your failures and the order in which and circumstance's of each event. When you call for the next appointment tell the service company you need an experienced tech because of "intermittent problems" this catch phase will usually get their intention and they would be more likely to send their top tech.
As an experienced HVAC professional of 25 years there are some thing a home owner should not do and mess with a gas appliance is one of them.
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snipped-for-privacy@gmail.com wrote:

You're right CSSTouffer.....
The little girl who answers the phone will make every effort to relay the O.P. long story.... and she'll make sure the service dispatch routes the most experience repair guy out that way. LOL
Unless the O.P. is dealing with a smaller M/P company which then all that *would* happen. - Maybe - :)
--
Zyp



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Your right Zyp I wouldn't relay the info to the dispatcher except as an intermittent problem the history needs to be conveyed to the tech.
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On Mar 22, 12:33 pm, snipped-for-privacy@gmail.com wrote:

If your having problems now since the basement has been dug out you may need to check all your low voltage wiring. If you had no problems before then I would suspect some wiring had gotten cut and could be grounding or shorting out.
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