Goodman Furnace GMT070-3B Four Blink, Open Main Limit Switch

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Old 02-20-19, 06:16 AM
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Goodman Furnace GMT070-3B Four Blink, Open Main Limit Switch

So I've got a furnace that has been baffling me.
It stopped working and it was only throwing the code of 1 blink which meant the start up failed. I cleaned the flame sensor and it came back on and everything was fine for a few hours. Once it stopped working again i ran down there to find out that it was cutting out with 4 blinks, meaning open limit switch and then after 2 minutes if would try and restart and then it would throw the 1 blink. I had a new spare flame sensor so I put that in and after it would fail after with the four blinks and then go to restart, it would still throw that 1 blink code of failed start up.
I will say if I turned the furnace off and back on immediately the limit switch was opening, it would start back up and run find for another 10-15 minutes.
I used my multi-meter to determine that the high limit switch was opening meaning that it's getting too hot, I pulled the filter and it would still overheat. I inspected all of the vents and they were not blocked and wide open. This is in a Duplex so I went next door and grabbed the upper limit switch off that furnace and switched it out. Still the same issue so I ruled out that limit switch. I tool the blower out last night and the squirrel cage was actually very clean. When it does fail from over heating, it will blow only cold air for some time afterwards. My plan today is to take apart the ducts and clean them. The cold air return is literally straight above the furnace and there's nothing blocking it with good air flow. Also, when i took the blower out, i stuck my head in there and saw that the heat exchange was very clean as well.
Am I missing something here? I don't have a manometer but I'm temped to turn the pressure down on the manifold to see if I can get it to run a little cooler.
Last night I sat in front of it after resetting and it ran non stop for almost and hour, burners on the entire time while the apartment went from 60 to 68 (thermostat was turned to 75 at this time for testing purposes) but then the same issue of the four blinks/open limit switch.

Anybody have any advice? Normally I'm the one giving advice to friends on furnaces but this one has me stumped.
 
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Old 02-20-19, 07:24 AM
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You should clock the gas meter and check manifold pressure.
DO NOT just blindly start messing with manifold gas pressure without having a measuring device.
 
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Old 02-20-19, 07:53 AM
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I am not going to change the manifold pressure without a manometer, I already have one on order. I'll clock the gas meter today after work. What do I do if the manifold pressure is good but the clocking test is high? Change the gas nozzles? I think they're size 43 on this unit.
 
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Old 02-20-19, 07:58 AM
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If the manifold pressure is correct and the gas meter clocks ok, you’ll have to start looking at heat rise across the heat exchanger and airflow.
Most natural gas furnaces aren’t permitted to be derated.
 
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