I have a Goodman model gmp 125-4 gas furnace. The code 4 limit switch code is blinking. I reset all of the heat limit switches, and checked the resistance across the series and read a 0.3 ohms. I know these are working. I turned the power back on after 30 min, and the same error code was being sent. What am I missing?

When we had our new furnace installed a few years ago we had a similar problem, after many calls and attempts to fix it, it turned out to be a wrong part having been ordered and installed on our particular gas heater. Haven't had any problems with it since. Ace

I'm sorry to keep saying this, Goodman/Janitrol GMP series has a major problem with their heat exchanger design, there are rivets that bust and draw flue gasses into the indoor air. This stream of heat, in severe cases, cause limit problems. Please call a pro.

I've been poking around and probing with my multimeter the series of sensors ends at a black box on the gas valve, a second wire is connected to that little box and is grounded to the burner enclosure. There is no wire back to the little cumputer board, how does the computer know that a switch is open. Also, I found out that an 8 volt spike goes across that wire for a split second when I apply the power, then no voltage.

The rivets aren't busted heat, I checked those already. Thanks for the reply though

I don't have a goodman schematic in front of me at the moments But I believe the main limit wires are red and the roll out limits are purple. The gas valve is connected in series with the pessure switch, so I'm not sure what wires you are speeking of. Take 1 wire off each limit as you check for continuity through each limit.

Right, I did that as well, and I checked each limit separately while they were unplugged, and I bridged around all the little sensers. The wire that connects all of these wires in series is purple. I saw the two red wires that run in the heat exchgange area behind the gas valve. If those are the wires to the limit switches, then I've been checking the wrong wires. What do the roll out limits check?

The part that goes into the heat exchanger is the main limit. The others tripp and need manually reset if the flames ever burn outside of the heat exchanger, say if the draft inducer stops when the flames are lit. Main limit become weak and don't close on their own again if they open to many times.

OK how do I close them?

To try and reset the main just remove it and tap it around until you get continuity. Put it back and try the furnace again. You should replace it or it will just do it again. You should also find out why it opened in the first place.

OK, thanks, I'll give it a try

That's not it heat, the resistance on that sensor is o.3 ohms, what else could it be?