Is it the flame sensor?

This is a Luxaire forced air gas furnace. Everything appeared to be working fine for the last couple of months. Nevertheless, we got a tuneup service done because we wanted to make sure that it was performing well (we recently bought this house). The serviceman gave it a clean report and said everything was fine.
On the first cold day after his visit (four days after the service visit), the furnace started acting up. This is what happens:
1. There is a click in the gas control unit 2. The flame starts, and it looks clean (ie blue with very little yellow), at least from what I can see through the inspection hole. 3. A few seconds later the fan starts 4. A couple of seconds after the fan start, I hear another click, and the flame goes out. 5. The fan stops after about one minute. 6. After about two or three minutes, the cycle starts all over again.
As a result, there is hardly any heat. I figured that this frequent on-off cycle every 3 minutes is not good for the unit, so I turned the whole thing off. It has been very cold, so this problem needs to be fixed soon.
I find it very strange that the problem started on the first cold day after the service visit. I called the company that serviced it, but he said there will be a service charge for him to look at it again unless the problem can be traced to his work. Of course, since I don't know much about furnaces, I can't prove that it was his fault.
From the little I know, this sounds like a problem with the flame sensor. If it is indeed the flame sensor, can I reasonably expect that he should have inspected that during the service? I am inclined to think that the furnace would be working fine today if I had not had it serviced.
Any advice on how I should proceed?
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There is a school of "thought" that feels things shouldn't be tested, like turning on a light, because THAT might be the last time the bulb works. Doubt that anyone can predict when a thermocouple will expire by examination by eye, my thought on the failed component probability. Taking a shower one morning the water was atypically warm instead of hot so it was pulled so it could be matched and HD had a match so wife had hot shower, alone. They fail frequently and are not pricey.
On 9 Nov 2003 09:55:04 -0800, snipped-for-privacy@myrealbox.com (Andrew Sarangan) wrote:

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Andrew, To me, it sounds like the person who came out to service your equipment was a technician, meaning that he was looking for a problem and if you had a problem at that time he inspected it, he probably would have found it when he inspected your furnace and would have notified you of the problem. If the person who came out to check your unit was a salesman disguised as a technician, I'm certain the salesman, disguised as a technician would have found several very expensive problems at that time and would have tried to separated you from some of your most favorite dollars. Lucky for you, it turned out to be a technician and not a salesman.
Jabs

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Get the parts book out find the flame sensor indicator, if its easy to remove shut off the power to the furnace remove the sensor wipe it off with some super fine steel wool, re-install turn on the power, turn up the thermostat and see if your problem is solved.
I had a Comfortmaker, 3 times a yr I would have to wipe off the sensor, the furnace would attempt to fire up then shut down.
The repair tech said to remove it and wipe it off with a dollar bill and that would be sufficient
Tom

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Ummm - wouldn't a $3 bill work better ?
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"Henry77" <:-P> wrote:

Henry, I'm sure it will work just as good. It kind of reminds of the time some guy asks me if I could change an $18 bill for him. I said "sure, what would you like? three sixes, six threes or two nines?"
Jabs
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Horseshit. He had someone out for a fall check and until provin different we'll presume he checked the safeties, filter, temp rise, co's. We can also assume at this point this guy never reported any problems and it never acted up during the check. I've seen a number of flamesensors that don't look coated but are just enough to intermittently interfer with the rectification process. If he had cycled the power it more than likely would have fired normally. But the percentage of those is very small that actually have a problem but do not exhibit it during the check. And that's about all you get on a Fall Check Special, anything beyond checking is extra.

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Andrew.
I hope you reply what the problem was concerning your furnace when you get it repaired.
Thanks Tom

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This is Turtle.
What this sounds like is the heat antisipator on the new thermostat on the wall was not set to .4 or so and still set at 1.2 or so.
Talk to the person who installed the thermostat to check on the problem. I get to see this same thing as you say 3 or 5 times a year but most are installed by the customer. Now some are installed by the tech working on them.
TURTLE
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