I have no idea what the actual model is, but from what I'm finding the code 21 is a gas heating lockout. I need to know how to fix it.. I took the bottom panel off and there was a black switch under it that popped out. When it did it completely killed the cycle. If I flip the on/off switch it will kick on, takes a couple of minutes to get started but it kicks. Back to the black switch under the bottom panel door... I taped it down. The heat seems to be kicking on and off just fine now. I have no idea what that means. Can someone help me, anything would be helpful. Thank you.

Could be a bad gas valve. Did you put the cover back on and still running OK?