I'm having a furnace problem. I turn the furnace on. The draft inducer
runs, the igniter lights up and then the gas goes on. It stays on just
about till the blower starts and then the gas goes off. The blower
keeps going for a little while then shuts off. I had an extra
thermostat and changed that out with no change. I cleaned the flame
sensor. The furnace has been running fine for years until this
happened today. Anybody know what's wrong?
Shutdown at that point in the sequence sounds like flame sensor.
You can put in a new flame sensor easy enough.
Might be bad, and they're cheap.
You cleaning it with fine steel wool?
If a new sensor doesn't work, bite the bullet and get a pro in unless
you want to fart around for days in a cold house.
If you do that look around on the net for troubleshooting your model.
Could be the motherboard.
When I had that happen cleaning the flame sensor fixed it for a period
of time. Had to clean it a few times a year.
A couple years ago the fan relay went bad on the motherboard, so I had
Came with a new flame sensor.
Haven't cleaned it in the two winters since then.
The original that came on the new furnace wanted frequent cleaning
right off the bat.
Rather than just replace it, can't he test it by bypassing it. If
it's normally closed during operation, bypass the sensor with a jumper
wire whereever it's convenient to attach it, with alligator clips on
the ends maybe. If it's normally open (I haven't worked on a gas
furnace, but I doubt it) just disconnect it, at the right time if
Even if it were free, it means interrupting his work to go to the
store. IF his testing gives uncertain results, he can guy one then.
Brand, model, approximate age would be very helpful. What do you
mean "it stays on", the igniter or the gas? It sounds like it could
be that power is removed from the gas solenoid when the blower comes
on, maybe a bad connection somewhere.
Thank you for your replies.
I cleaned the flame sensor again and checked continuity on the flame
rollout switches. I have continuity through the main limit switch but
still have not been able to check the auxitllary limit switch. I'll
have to take allot of stuff off to get to it and don't want to try
till I get a new flame sensor tommorow. The furnace was made by
Goodman Manufacturing Company and is a model GMP100-4. It's about
fifteen years old. It's the gas that goes off around the time the
blower starts. I've timed the burn time from 10 to maybe 20 seconds.
I can't find any way to reset the thing after it goes off. I turn up
waiting more than an hour before it will fire again. It does have an
LED code but it doesn't blink but stays steady which is the all OK
code. I'll try a new flame sensor tommorow and if that doesn't work
I'll either call somebody or try to get to the aux limit. The board
has to come off to do that.
This morning I went down and turned the furnace on to see if my latest
cleaning and connector wiggling had helped anything. It did the same
thing and the gas shut off at about 12 seconds. I forgot to turn it
off again. About an hour later it came back on while I was upstairs. I
waited for it to go off but it didn't. Now it seems to be running
normally. ..Great an intermittent problem..this will be easy to fix.
To reset the furnace from lock-out, try this: turn the thermostat down to a
temperature cooler than the current inside temp. so that the thermostat
isn't calling for heat. Turn off the power to the furnace. Now, turn the
power back on. Turn up the thermostat so it's calling for heat. Does this
You might be right, but I don't know how to bypass it, and don't want
my advice to make anything worse.
I just hardly do anything electric and use the "pull and replace"
method for electric if the part is cheap and I suspect it's the cause.
I do know a few things to keep me safe, and that's it.
Last time I even put my ohm meter on anything was testing some fuel
injectors. The results were "inconclusive" so I just nought new ones.
That worked good.
Besides, see below.
Based on what he said, it will.
I dealt with the exact same problem for years, but cleaning the flame
sensor always got the furnace going again for awhile.
A new flame sensor stopped the problem entirely.
Problem is, the sensor came with a new motherboard, so I'll never be
sure which cured it. Just don't know.
I sometimes had that intermittent problem too.
I would hear it do the failed cycle, but it would "cure itself" before
I got to cleaning the sensor.
One thing I didn't mention is Bobo is to make sure he's cleaning the
flame sensor, and not a rollout sensor.
Didn't want to insult him.
My Rheem manual had a variety of possible control systems, one with
2 flame sensors.
I must have cleaned the rollout sensors 6 or 7 times before I caught
on to the fact that I had 1 flame sensor, and was missing it.
The flame sensor was pretty well hidden.
What fooled me at first was after cleaning the rollout sensors the
first couple times, it worked! Then it stopped working, so I dug in
some more and found where I was wrong.
Another example of an intermittent problem confusing the issue.
But I like your idea of bypassing for a test.
Next time I run into such an issue I'll try jumping a component if I
think I won't zap anything.
I replaced my HSI once, and have a spare on hand.
When I was looking up HSI - never heard the term - I ran across this,
which is a pretty good basic explanation of common furnace light off
One thing isn't clear about Bobo's problem, and that's how long the
furnace fires before shutting down.
My Rheem would only fire for a few seconds when the flame sensor
Bobo seemed to indicate his is firing much longer.
If that's true I take back everything I said.
Yes, the gas stays on a little longer in mine, and the duration
varies from say, ten to twenty seconds. After it started working again
this morning I turned the heat down before I went to work.. When I
returned home the problem started again. As it seemed to work fine
when there wasn't much time between burns I thought warming it up
might help so I put an electric heater in front of it for awhile and
it's running fine now. I mailordered a flame sensor when it was
running good this morning. I hope that does the trick. The mailorder
place has boards for $125. Hopefully I won't be sorry I didn't order
one. I sure have learned allot about modern furnaces the last couple
I think the flame sensor produces voltage when hot so it would be a
tough thing to simulate.
I'm no expert in this area, but here are a few thoughts.
The rooftop gas hot air heating unit where I work was doing the same thing.
It would run for a brief period and then shut off. It could be reset and
restarted, but then it would do the same thing. It turned out that it had a
cracked heat exchanger. It is also a Goodman and is about 12 years old. In
our case, the local gas company was able to diagnose the problem, and the
technician said that there was a period of time back then when Goodman made
models that had heat exchangers that tended to crack after about 10 years.
He said they later changed the design of the heat exchangers which makes
them last longer.
Since a cracked heat exchanger can send carbon monoxide gas into your home,
be sure you have a working CO detector in your home.
In our area (Southern New Jersey) the gas company (PSE&G) will come out and
check out heaters and other appliances, and they do heater, HVAC, and other
appliance repairs. The have crews working both day shift and evening shift.
The way they work it here is that they come out for free, and if they can
tell what the problem is (which they almost always can do), they tell you
how much it would cost to have them fix it. Then we have a choice of either
having them fix it or having someone else fix it. There is no charge for
the visit if we don't have them do the repair. When we call and say we have
no heat, they almost always come out the same day withing a few hours. If
your gas company has any kind of similar policy, that would be an easy way
to get your problem diagnosed and probably fixed. They put the repair bill
on your gas bill if you decide to use them to do the work.
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