My 8 yo Frigidaire gas furnace is on the fritz. It does come on and funs for short cycles but it turns off well before it heats enough to trigger the thermostat, then it turns back on and runs again... over and over... Between the on/off cycles the red light on the board is flashing continuous which the diagram says indicates "False flame or gas valve relay shorted". Anyone know what that could evolve?

Thanks.

Clean the flame sensor with fine sand paper or steel wool. Below are a few picks of what it may look like. Extra care is needed when working aroung the ignitor since the ignitor is fragile.

Thank you. I did that earlier today. Before I cleaned the sensor, it was actually running worse than it is now. The igniter would start and as soon as the gas turned on, the gas lit briefly and everything would shut down. Now it will run for five minutes, maybe a little longer before it shuts off. Do you think it's possible that I just didn't clean the sensor well enough? I used a wire brush and was pretty gentle with it. Thanks again

Fine sand paper or Steel wool or a Scotch Brite pad works the best. You can use a meter if you have one to test the signal. Sometimes the leed from the sensor to the board looses contact OR becomes internally damaged.

I had an emergency but I'm still having this problem.

Recap of everything I know so far-

Heat comes on normally, runs for 3-4 minutes, shuts down. Red light starts blinking continuous at that point (which indicates - false flame or gas valve relay shorted). The system resets itself and starts up again as if normal after about 4 minutes, then over and over.

I've cleaned and inspected the flame sensor.
Flame sensor test reading 3.0 uA DC

How would you proceed from here?

I realize now that my furnace is overheating for some reason, but I can't figure out why. For testing purposes, I waited for the gas to ignite in the furnace, then I turned the gas down slightly (maybe an 8th of a turn) from the shut off valve (the main valve outside the unit). It ran until the thermostat was satisfied and shut off like it should do normally with no fault but once it restarted, there wasn't enough gas flowing to ignite all the burners. The first burner lite and that was it and it shut down when the flame sensor didn't get the flame etc... again, If I turned the gas down enough for it to not overheat, there wasn't enough gas to restart.

Earlier before I decided to turn the gas down slightly, I disconnected the exhaust vent, inspected it and it was clear of any obstructions. I removed my filters for testing and ran the furnace without them and it overheated. I actually cut a large panel out of the trunk (I cut a taped joint and re-taped it afterwards) directly above the furnace thinking that that would allow as much hot air flow as possible (in case there was some obstruction in the ducts), it still overheated so I figured there weren't any obstructions in the ducts causing the heat. My blower and exhaust seems to be working properly. What other causes of over heating could there be? Wonder if it would make any difference if I adjusted the gas flow from the electronic gas valve?

I also wonder if even though the induced draft blower seems to be functioning properly, from what I can see (the fan's working, it's turning on and turning off when it should)... but there might be some internal problem with it that I can't see. I'm not sure how it works on the inside.

I know my furnace is overheating #1, and there are no obstructions anywhere (I've checked everything) and it isn't a sensor malfunction (I don't think), so I figure it could be that the blower isn't working efficiently enough, or the induced draft blower isn't either or there may be some problem with the gas valve. I can't think of anything else that would cause this.

In my manual, it mentions some way I can change the wires on the board from the blower and change it's speed but it doesn't say how that works exactly. Maybe the blower isn't turning enough RPMs, but it seems to be the same as it always was.

This is a diagram of the blower wiring on the board. The manual diagram is on the left and the way mine is wired is on the right. Wonder why there's a difference? On my board, the blue wire is on the EAC tap (in the manual it's on the M1 tap) My orange wire is on my M3 tap (the manual shows orange on the M2 tap) the red and black wires are the same for both.

http://www.imagechicken.com/uploads/1263499452007290700.jpg

Here's what my manual says about blower speed and heat etc
“blower speeds will increase the temperature rise and
Higher blower speeds will decrease the temperature rise.
The furnace is equipped with a multi-speed motor. Heating
And cooling speed selection is made by moving the leads
On the integrated control located in the furnace. The wiring
Diagram on the furnace and Figures 27, 28 and 29 show
The speed taps for adjusting motor speed.” It doesn't really say how to change the wires to increase the speed (if that's what I need to do).