Carrier furnace trouble

[pstgh]

So the good news is we don't need heat right now!

Have a somewhat older Carrier gas furnace (if you need the exact model etc...I'm happy to get that) that is about a 3 ton system- circa 1993 or so. When the t-stat calls for heat, the induction fan/vent comes on, and stays running...no power to the electronic ignitor. I've typically only had to replace the ignitor in the past, but it is fine (I've taken it out and bench tested it- and it glows right up when powered). So now I'm wondering what the next best step is to diagnose my problem- I've pulled the flame sensor, but it doesn't really look like anything is wrong with it- just a simple bent metal rod that sits in the flame (when on). Any way to test those?

Any thoughts from here would be greatly appreciated.

[junkcollector]

So now I'm wondering what the next best step is to diagnose my problem- I've pulled the flame sensor, but it doesn't really look like anything is wrong with it- just a simple bent metal rod that sits in the flame (when on). Any way to test those?

I believe you can test a flame sensor by disconnecting one of the leads and connecting a multimeter set on milliamps in series with it. (If there are no amps flowing the sensor is bad or there is a bad connection) Usually though, just do a visual inspection. If it looks like it has a lot of buildup on it or it is dirty in any way, buff it clean and shiny with fine steel wool. If the flame sensor is clean, I would suspect that there could be a problem with the control board.

[pstgh]

Thanks- the flame sensor is actually connected with just a single wire- almost like a temp gauge or something. I guess I'll just brush it with some swool and put it back. Regarding the control board- should I just pull it and replace it or is there a way to identify a specific problem that can be fixed?

Also, I'm wondering if I might be missing something with the ignitor- I may put some volt meter leads onto the ignitor plug to confirm that power is not being sent out there. Do you know about how long the control board waits for the induction fan/vent to clear the stack? When I took the ignitor into the parts shop, they hooked it to some raw power and it fired up, but if the control isn't sending the same amount of high voltage, it may be broken, but appear not to be from the bench test? Maybe not, just not too sure.

Again- thoughts appreciated.

[pstgh]

OK! Don't all chime in at once now!

I need to go troubleshoot it some more (like I said earlier, I'm happy it isn't cold right now!). I'm planning on confirming that the T-stat is good, but I believe it is fine since it triggers the induction fan to come on. Then I need to check the switch that tells the board it is ready for ignition. Then, if not that switch, I guess it's a new board for me!!

Thoughts? Questions? Thanks?

[fordrules]

Is there power coming out of the pressure switch, and are there any codes flashing? If just the ventor is going with no ignitor warmup, I would be checking safeties: rollout switch, hi limit, pressure switch etc. To test an ignitor, I usually ohm 'em and amp 'em. Should be around 100 ohms for a w style ignitor, and around 3.5 amps.