carrier 3 wire pilot gas furnace random lockup, pls help


Old 11-15-17, 12:30 PM
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carrier 3 wire pilot gas furnace random lockup, pls help

hope someone here could offer some hints what is next for me to try. Sorry for long description

old Carrier gas 394GAZ048125AAEA, 3 -wire-igniter furnace (bi-metal flame safety switch) . Several contractors were unable to pin point the problem. The problem is it randomly locked up. it works for few days no problem. but every few days, it would lock up like this: call heat, sparked, pilot lighted up, main burner light up. At this moment, if the spark continues, then the whole thing is shut down and locked up till I flip the power switch off/on; If the spark stops, everything goes on no problems for heat cycles.

here is what has replaced so far: new 3-wire safety burner LH680005, new spark electrode, igniter/lockout control grey box. 2 limit switches are checked close all the time. new thermostats, new air filter, new red spark wire. All the possible parts are replaced suggested by contractors. so I run out of any ideas as what else to check or replace? seems all the relevant parts are replaced with new ones.

in short, the problem is if the spark continuous after pilot light up followed by the main burner lights up, it will shut down and lock up for good; if sparks stops after pilot on, all are normal. This lockup occurs very few days. most of time the furnace works fine.

pls shield some lights on the problem, I spent so much time and are unable to get it fixed.
thank you in advance for any comments
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Old 11-15-17, 02:12 PM
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In an attempt to simplify the problem, the question would be, what would make the spark continue? It's supposed to spark once & that's it, right? Could something as simple as a bad ground cause that to happen?
Old 11-15-17, 07:01 PM
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thanks for your suggestion. I measured the ground at all the locations from the furnace diagram such as ignitor grey box ground, control board ground, and power supplier ground. they are good and tight.
Old 11-15-17, 09:57 PM
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Welcome to the forums.

Not too much info available on that almost 30 year old furnace.

Apparently that furnace uses a pilot flame sensor. It may be a combination igniter electrode/flame sensor or it could be separate. That rod may need to be cleaned or repositioned in the pilot flame.

Manual.... hvac/docs/1009/Public/06/II394G-25-2.pdf

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