So I have a weird problem with my goodman furnace. It's about 17 years old. The model is GMS81155CNA.

I installed a nest thermostat last year when we moved in replacing an older non programmable thermostat. When I installed the Nest there was a blue wire in the wall that wasn't hooked up to anything. I used that as my C wire and hooked that up to the C terminal on the board in the furnace. Everything ran fine all summer and winter last year, as well as all summer again this year.

In late october I had the furnace serviced as I did the year prior. The year prior I had a new blower motor installed. Everything checked out fine, the tech did his normal thing and we were good to go (tested before he left). Fast forward a few weeks and we had a string of colder weather...I noticed the furnace would go through it's normal process, but the flame would go out after about 5-7 seconds (sometimes longer, sometimes shorter). The fan would still kick on but would get cold air. Sometimes it would run for a bit, other times it would shut off after a few minutes.

I believe the board was replaced at some point as there was paperwork near the furnace for it. I looked at the LED and saw it was flashing 8 times. I looked at the paperwork and it said that either the C terminal isn't grounded to the frame of furnace or I have a bad tstat.

I'm trying to figure out what the actual issue is. It's very intermittent. I had the hvac tech come out and he cleaned the flame sensor... that helped for a week or two, then the problem came back. I changed the flame sensor out for a new one, but same issue still persists.

I did notice there are two other wires in the c terminal besides the blue one that I used for t-stat. Last night I took those out and re put them back in and retightened it. It worked overnight and again this morning, but I'm just waiting for it to fail.

I've attached some pictures of the board/wiring and the wiring diagram I have, I'd love to hear thoughts/suggestions. Do I really need to ground that C terminal somewhere else? If so why did it work all of last year and not this year. I did also recently replace the air filter so that is brand new.



If there are any other/better pictures I can add let me know.

 

Problem went away for about two weeks but is back the last two days. Same symptoms.

Having an HVAC guy come out tomorrow and I bought a new tstat just in case but this is super frustrating because it's so random.

 

You have a flame sense issue and a poor ground to the control board could trigger a fault 8 code.
It could be the ground on the control board.
It could be loose/corroded burner screws. Many times when I run across ground issues I install star washers on the frame, burner and control board screws to insure a good ground.

 

Where would I look to see if I had a bad ground on the board? I guess I'll just ask the HVAC guy tomorrow.

The flame sense issue is weird since I cleaned and then replaced the flame sensor recently.

 

You have a flame sense issue..... not a bad flame sensor.

The flame sense is a closed loop. It goes from the control board to the rod thru the flame to the burner ground from the burner ground thru the frame to the control board. If the loop isn't good. The flame sensor won't rectify the AC and produce enough DC current. The flame sensor is nothing more than a stainless steel rod that sits in the flame. They never go bad..... just get dirty.

A current meter is put in series with the flame sensor wire.
The current produced when the flame is present tells the status of the circuit.

 

Gotcha. Makes sense. I will mention the grounding issue to the tech tomorrow and gave him check the board and burners.

I just wish I could figure out why sometimes the ground is good and sometimes it's not.

Hopefully it's something that simple...I mean the led is saying bad ground or bad stat so has to be one of those.

 

The gas enters the burner from the right side. The igniter is on the right side and the flame sensor is on the left side. The flame must light and transfer thru every burner to the left one in several seconds. If it doesn't the cycle will fail. Possibly the burners need to be cleaned.

On the control board..... the ground is the green wire on pin #8. Follow that and make sure it's solid.

 

The green wire is held down with a screw on the upper right of the frame of the furnace. You can kind of see it in one of the pictures. If that is the wire it looks like it's grounded pretty good and tight.

In any event thanks for the info I'll report back after the HVAC tech comes tomorrow.