Furnace help

Hello all,
I tried this over at alt.hvac and was redirected here:
I have a Janitrol (I know, it came with the house) gas heater.
A couple of times in the past, it has stopped working and the tech came over and replaced a flame sensor.
One of the times, the tech simply sanded the sensor and said this would extend the life - he said you could maybe get one or two sandings out of one, and get an extra season (or worst case, get you through the night, or day until the tech could come out with a new one). He kind of showed me how to do this as I watched.
Each time, the heat has had this problem, it started igniting the burners, which burn for about a minute, and then go off, presumably because the sensor is not seeing any flame.
So a couple of weeks ago, it started again (it's probably been 1.5 to 2 winters since the guy was last out, but I can't remember if he replaced, or sanded last time). It sounded the same to me - I can hear them downstairs, and when I go up in the attic to check, I can see the flame come on, and shut off after about a minute. Once this starts, it just continuse this cycle every couple of minutes. So basically I get lukewarm air blowing out the vents.
Now this time, it came on more gradually (though because I'm more aware of it, I might have just caught it earlier on - in the past I didn't know until I woke up and it was 60 degrees in the house), and if I cut the thermostat off, and then back on, it would blow heat the first cycle. After that, lukewarm air. However, it ran last night and though the house got down to 65, when I woke up this morning it was blowing hot - so it must be coming on intermittently still (BTW, it's been warm here, so we haven't needed to run it, plus the holidays, so I've been putting off calling the tech - even tonight, at 2:00 AM which it is now, it has not come on (in fact we just had a thunderstorm in the middle of winter! that's highly unusual).
So I went up tonight, took out the flame sensor (I cut power of course) and took some 320 grit sand paper (which is all I had) and lightly brushed it (no pressure by hand, just from the paper) like the guy showed me.
Now I have "repairer's remorse".
I read on the oh so accurate internet that I should use emory paper or light steel wool to do this.
Anyways, my questions come down to this: How long should a flame sensor last (I seem to be getting 2-3 seasons out of one)? Is a flame sensor something I should be buying and installing myself? If so, is self-diagnosing this problem based on my previous experience wise? Or, should I just put one in every season regardless (as if I was putting new batteries in my smoke detectors every daylight savings time change?)? They seem to be ranging from 10 to 20 bucks, or as high as 134 bucks - so something funky is going on there - I realize various models will vary in price, but are these things a "maintenance" item, or something I should be leaving to the pros? Will having a service agreement with a tech include cleaning of a sensor, or will something else they do help to extend its lifespan? (I would assume they'd replace it if it fails, but is it something they can test and will replace or recommend to replace on a regular check up?)
Finally, If I've sanded the sensor, and the heat seems to be running normally (if it ever actually gets cold here in the next couple of days - it should be in the 30s at night now - it's not), should I get the tech out here anyway (OK, I know it would be a good idea) or is this something that I can blow up the house with. That is, if I sanded a sensor too much, would it see flame when there is none, and let the gas come out, rather than just always assuming there's no flame and cut of the gas - which is what it's supposed to do - and which is what happens when there's carbon buildup - sees no flame even though there is some and shuts off as a safety precaution. Seems like it's a device that if it senses no flame, either because there is none, it's covered in gunk, or the wire's disconnected, it will shut off the gas. Right?
Thanks for any assistance you can offer, Steve
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your theory of its operation sounds generally valid. but there are too many variables in the wide world of flame sensors and the different heating devices and the countries where they are used to give you an okay. here's why: in my case the pilot flame must specifically be adjusted to match the illustration in the gas boiler manual and the replacement pilot instructions. otherwise the pilot itself must be cleaned and adjusted. the boiler main burners requires correct adjustment and positioning of the gas pilot flame. now you need to get into the replacement gas valve's manual to see what's wrong. if in my case the pilot was low. my flame sensor spark ignites as well as senses with a sparkplug-like wire back to the boiler control board. when it doesn't work properly it may be the flame sensor, its dirty tip, the voltage it makes or doesn't make as it should, how the gas valve's pilot responds to the control board's directions, how the main valve behaves , and how clean the burners and their crossovers are in your cold or hot or damp or dry or clean or dirty appliance room's environment.
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Steve Latham wrote:

Hi, My 11 year old Carrier furnace had one HSI replaced, main logic board repaired by me, flame sensor is still original one. Is the sensor in good position to sense the flame? When it acts up can you fetch trouble code? Usually it retires 3 times and if no go, it goes into lock up mode. You can reinitialize logic by powering on/off. Tony
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OK. That's telling. If the flame sensor is 11 years old and still going, my having to replace one every couple of seasons is indicative of another problem, a tech who wants to nickel and dime me, or both.
Is the sensor in

I don't know how to do anything like that. Which probably means I should stop screwing with it.
Usually it retires 3 times and if no go, it goes into lock up

Yes, that's what it seems to do. I don't know exactly what lock up mode means, but it will try, and then sit for a while (running the whole time) and then it seems to me it may re-try another set, and then wait again.
You can reinitialize logic by powering on/off.
Well, it will start and run properly if I turn it off, wait a while, and turn it back on. But then it only goes through one cycle (appx) before it goes into the re-trying.
Thanks, Steve
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Damn Steve, You are making way too much of this flame sensor deal. Its a flame sensor, PERIOD. Its a metal and ceramic rod of which the metal portion is stuck in the path of the flame. It sends a signal (heat converted to electrical) to tell the gas valve to open. They are subject to moisture and contaminents. They are also subject to poor manufacturing just like everything else made in Taiwan, Mexico and the Phillipines. You clean it once a year with the "tune-em-up". If it gives you a problem inbetween, replace the damn thing. Simple. If you dont trust your "tune-em-up" guy, replace him too. Hell, maybe its just you and he will be happy you did replace him. Bubba
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Steve Latham wrote:

Is it possible your flame is generating excessive soot?

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Robert Green posted for all of us...

RIP Don't know about the others in HVAC.
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On 1/9/2015 3:24 PM, Tekkie® wrote:

IIRC, wasn't that C. Terry Lee, from Shreveport, Louisiana? Seemed like he knew what he was doing.
I used to call alt.hvac "alternating havoc ". With good reason.
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