Hey, I have a goodman gmp-100. I got the 4 blinking light indicator a few days back. Open Limit switch. When i turn the heat on, the burners go for about 2 minutes but the an does not kick on. It finally kicks off and then blinks. I tried turning on the FAN ONLY and it came on no problem and was pushing air thru the vents. So the blower works. I checked the 3 limit switches and the buttons on them are flimsy, not stiff. Any ideas. I have a multimeter, but am unsure how to check the limits

This is a dual-fuel heatpump-gas combo. I noticed this morning (first time temp low enough for the gas to come on) the heat pump started first, after a couple of minutes it stopped. I then heard the familiar hum of the gas furnace. It hummed for several minutes, so I opened the cover. No flame, no fans, just humming sound coming from the top of the unit. After a while, it shut off. The stat timer fired it back up after 3 minutes, gas unit fired up perfectly. It seems when it switches from the heat pump, it doesn't start (just hums). But after cycling off/on, it works. I'm thinking control module, what do you folks think?