I have a Goodman gmp125-4 gas furnace. I've been having all sorts of problems recently. I've fixed most of them, but the last problem I had was a code 5 "false flame sensed". The chart inside the door said the most likely problem was a stuck gas valve. I assume this is the problem if the fire has been burning, and in the off cycle, the fire is still burning sending a signal through the flame sensor, the system shuts off. My false flame sensed was happening when the furnace was turned on. I hooked the multi-meter up to the flame sensor, and was getting 50 or so volts while there was no fire. I unhooked the sensor, and got the same reading from the line itself. If I wiggle the line around, the voltage gets down to about 10 volts. The false flame alert is cleared, and the light is constant. I grounded the wire for about five minutes, and the voltage went down. I got the furnace to fire up, but I don't know how long until this mystery voltage reaccures. I need to know what is going on what this voltage is, and how to fix it for good. I always thought that the voltage came from the flame sensor, and was created by the heated flame sensor. If you know please tell me.

The flame sensor always has a voltage to it, and can change by manufacturer. What it actually does is rectify a current. In other words. When you pass an A/C current through a flame, the flame rectifies it into a D/C current. Like a diode does. If the flame sensor shorts to ground an A/C current is passed and the PC board knows it. Only when a D/C current is sensed through the flame sensor does it believe there is a flame prescent. Very safe, Huh? The ceramic mounting of the flame sensor could be cracked, the gas valve might have stuck open, very rare, unless it got water damaged. Or the PC board it going bad.

I find the flame rectifier interesting. I know flames are conductive. They can also be used as a audio speaker.

Thank you again heat.