My mother in laws Goodman 80% furnace displays a 4 blink error code (Hi limit open) and restarts several times per day. I'm trying to help her out because she is living on a widows social security and doesn't have the cash to call a repair service.
About once per day it locks up with error 1 because it fails to ignite after 3 tries which I believe happens after error 4 causes it to restart. After a few days of troubleshooting and reading various web sites I have replaced the Hi limit sensor (300F), and the control board, and the flame sensor. The air filter is a brand new cheap filter, both cold air return vents have been vacuumed out and all heat vents are fully open so there are no air restrictions outside of the furnace. We looked at the flue on the roof and there is no debris covering the holes. If the flue is plugged I would think we would feel a lot of air coming in through the hot water heater vent while the inducer is running, correct? I think I have covered all the easy fixes so far and wasted 150 bucks. I think all that's left to do is check the evaporator for debris and the heat exchanger could have a crack? Does anyone have any other ideas for Hi limit open failures? Also, how do I access the evaporator?

 

Welcome to the forum!! To better assist, please post the model number of the furnace.The furnace is overheating very likely because of inadequate airflow through the furnace. The most common reasons for this a 1) a plugged filter 2) return air vent covered over 3) too many warm air vents closed 4) plugged air conditioning evaporator coil a 4) dirty fan wheel 5) blower is not providing enough air flow 6) blocked vent pipe. You have already eliminated the first three on the list.

 

It's Model #GDS80904BXBA.

 

There could be something inside the pipe such as a bird's nest, wasp nest, dead animal or etc. further down from the opening on the roof. As far as the evaporator coil, do you have air conditioning? If so, the coil will have a pvc drain line coming out of it and a copper tube going into it. You will have to remove the access panel to gain access to the coil. If the coil gets clogged up it is just like a dirty air filter and will restrict air flow causing over heating. If possible, post some photos of the furnace.

Do you have an auxiliary limit switch shown in the attached parts diagram? It is rated for 120 degrees.

 

I can get some pictures on my way home tonight. It does have Air conditioning. A pvc and copper pipe pass into the bottom area of the furnace. They used to have several cats with long hair so the evaporator is probably clogged with dust and hair. I'm not sure how to access the evaporator. It doesn't look like there is a front panel in the evaporator area and the furnace is surrounded by walls on 3 sides.

Would the auxiliary switch cause error 4? I haven't opened the blower chamber so I don't know if it has that switch.

 

Your furnace is a downflow model. You should have an access panel in front that just has a few small screws to remove, Cat hair will definitely clog up a coil. On some furnaces an open auxiliary limit will throw the same flash code as the primary. Here is a link to the installation manual for the furnace:

http://www.alpinehomeair.com/related...structions.pdf

Page 26 in the manual discusses an auxiliary limit switch and the wiring diagram shows there is one..

 

According to the manual you gave me the auxiliary switch can cause error 4. I'll try to open the panel tonight and I'll replace the switch. If it stays working then we are good, if not I'll have the heating service come out for their $89 cleaning and tuneup special. According to their website they will check for leaks, clean the blower and like 28 other things. I'll ask if they can clean the evaporator (for a nominal fee I'm sure).
If the heat exchanger is cracked, can it be replaced or is that the end of the furnace?

 

If the coil is not terribly plugged up and you can gain access to it, you can try and clean it yourself. As far as the heat exchanger it should be under warranty for the part but not the labor.

 

I believe the evaporator is in this box. How do I get this cover off with all the pipes passing through it?

I disconnected the auxiliary switch while the furnace was running and no error code was displayed but the gas valve was disabled and then the furnace produced error 1 after it tried to ignite 3 times. One of the wires for the auxiliary switch connects to the gas valve so this makes sense. But It seems like a bad design with the motherboard not knowing that the gas valve is disabled. They do see error 1 maybe once per day so maybe the temperature in the blower exceeds 120 because the temperature near the hi limit switch is near 300. I jumpered across the auxiliary switch and error 4 still happened every 3 or 4 heat cycles.

I proved last night that jumpering the hi limit switch for 10 heat cycles prevented error 4 from happening. I removed the jumper afterwards for safety. I think this points to an air restriction inside the furnace. (remember I already replaced the hi limit switch and the control board and the air filter and vacuumed the return vents and made sure all heat vents were open)

 

It looks like you can remove the access panel by removing the screws around the perimeter of the panel and carefully sliding it straight back and then down. The white zip tie on the insulated pipe will have to be cut to slide the panel down. I believe you will find that evaporator coil clogged up as it appears there is a lot of dust on the panel and around the seal. The best way to test the limit switch is to connect the two leads of the switch. Then call for heat and test for continuity across the terminals of the limit switch. You can see it go from closed to open when it trips due to overheating. It would be beneficial to stick a probe thermometer into the discharge duct to monitor at what temperature it opens.

 

My picture doesn't make it very clear that the panel is already practically touching the floor so I can't move it down. How would a tech normally handle a panel like this? Would they cut slots for the pipes and slide the cover to the right or just cut the panel off and make a new one with slots?

I forgot to ask before but I noticed that the furnace doesn't always ignite on the first try. Is this normal? I looked at the igniter while it was hot and most of it is orange but the top 20% is more reddish which I think means it isn't as hot.

Thanks for all your replies firedawgsatx. I really appreciate it.
Dan

 

It appears the installation was done in that manner because of limited access on the sides. It is impossible to tell from the photo because we can't see the bottom. That panel obviously was installed after the refrigerant lines were installed. I would remove the screws and then pull it straight back as far as it will go and then see if you can drop it down. Normally, the furnace should ignite on the first attempt. I would check the ignitor to see if you see a crack in the element. Be extremely careful as they are very delicate and will crack easily. You can also ohm it out. At a room temperature of 70 to 77 degrees F. it should show between 30 and 300 ohms for your particular ignitor.

 

I found this pic on the web. It looks like someone added a cutout to make it easy to remove the evaporator panel. If I can't maneuver the panel out of my way, I will cut a notch like this panel. I already have a roto zip. I just need to buy a metal cutting bit for it.

I'll ohm out the ignitor on Saturday when I attempt to clean the evaporator.

 

I found the light at the end of the tunnel

The Solution:
I finally narrowed it down to a dirty evaporator coil. I snagged a fluke 179 meter with a thermocouple from work and placed it inside through the limit switch hole (I simply loosened the switch and slid the thermocouple inside an inch or two). As soon as the temperature hit 302F the control board shut the gas off and displayed error 4. I noticed the temperature dropped very quickly after the burners shut off. The switch was probably only open for a second or two and then it closed as soon as the temp dropped below 300.

The Fix:
Since I was fixing my mother in laws furnace my wife had no choice but to let me purchase a nice new Dremel 3000 with the extension handle and a metal cutting bit . It cut fairly quickly but it also created microscopic slivers. Note to self - wear gloves next time. I used the Dremel to cut out the panel around the 3 pipes that were entering the front of the evaporator box. Once the panel was off I used a vacuum and a small dish washing brush to clean off the thin felt-like layer of debris from the evaporator fins (always go up and down, never sideways). Replacing the panel was a bit of a nightmare. The furnace was a little wiggly without it and I had to move the furnace slightly in order to get the panel to fit. I started on the bottom screws first and just lined up each hole wiggling the furnace as needed. Not much quality built into that evaporator box!! All together I was on the floor lying on my side for several hours and my neck is killing me.

The Test:
After putting humpty dumpty back together again I measured the temperature behind the limit switch for 3 heat cycles and the temperature never rose above 224F. They haven't seen any error codes since I fixed it a day ago so I'm pretty sure it's good to go.

Tomorrow I'll summarize what I learned during this last week as a future reference for myself and hopefully it will help someone else so they don't make the same mistakes I made.