My 10 year old Goodman CMP100-3 come on, fan running, flames up for a few seconds, but won't stay lit. I have replaces the Flame sensor and have noticed that the red led on the HSI board stay lit. No flashing. Could my problem be the HSI board ($217)? Or something else?

You say you replaced the flamed senser. How long ago? Probably not a board problem, but its not completely out of the question. I would check to make sure drain system on the unit is not plugged up at all. It may be worth having a pro in to assess the situation, as it may be a simple fix and cost you less to have it fixed by a pro than the price of a new board alone. I personally never replace a control board until all other components of the furnace are checked and then Im sure it is the board. Its never fun to replace an expensive control board, just to find you still have the problem.

I just repalced the flame sensor yesterday and the igniter 6 months ago. I have a Technical back ground. I even checked vacume line, all the Flame out sensors that are in series (0 ohms. There no obstruction in the xhaust pipe. I feel confident I can fix it I just wanted some feedback before I spent $ 217 (cost) on a HSI board.

Okay. A plug in the drain system will cause one or both of the pressure switches to not operate properly. Does your unit have one, or two pressure switches. Either way, check to make sure neither switch is opening when the furnace fails.

That being said, it really sounds like a flame sensing issue. Even with a new flame senser, there may be an issue with the wire, Or as you suspect, the board. You can test the flame senser with a multi meter. You need to set the meter to ua dc and put one lead on the flame senser connection, and the other on the wire that hooks to the flame senser (with wire disconnected from senser). The reading on the meter should be 2 or over 2 while flame is hitting the senser. If it drop below 1 - 1.5, the furnace will work intermitantly, or not at all. If below 2, there is a flame sensing issue, whether it be the flame senser itself, the wire, the board, or incorrect possitioning of the senser, or lack of flame (low gas pressure can cause this aslo). Its tough to really give great advice without putting my hands on the furnace, but Ill do my best to help out. Let me know how the pressure switches and flame senser test out, and we will go from there. Good luck, and please let me know what you find.

PS. Also check to make sure the HIGH LIMIT is not opening when burners are burning.

Thank you for responding back. I did check those things and have a pro helping me. My problem is intermittant and I have been able to better asses my problem now that I have carefully watched the furnace. The bottom burner dose not always have a flame or has a problem in keeping flame up. The top three burners are find. However the Flame sensor is on the bottom burner. It will continue cycle and run its normal pattern until the bottom burner can substane adiquate flame so the flame sensor can tell the HSI unit Hey I'm lit. I did remove the pipe from the furnace and checked and cleand my nozels. I even unloosen the bottom nozel and checked it visuall and blew into the pipe to make sure no obstuction was in there. It appears more clearly now the my problem is in gas flow but see no adjustment on the regulator. Now I'm scatching my head whether to replace the regulator.

The pressure should be adjustable on the gas valve. Have your Pro friend get gas pressure readings on inlet and outlet of gas valve and adjust as necessary. It may also be as simple as a dirty cross-lighter bar on the burners.