I have high efficiency gas furnace about 10 years old.It is NUGM075EGB1 made by Intercity products corp.
The problem I have is cycling. In the morning the programmable thermostat call for a temp rise from 18 C to 21C. After about 30 minutes the flame turns off and so does the combustion fan. But the temp at the thermostat is only 19C . The duct temp is only 126 F .The main blower then turns off after 3 minutes even though the duct temp is still at 90F. About 2 minutes later the furnace starts its cycle again. If it was the upper temp limit it should not have turned off the combustion fan at the same time as the burner went off according to the installation book. I am stuck .Can anyone help me.Thanks Hugh

You should have a code flashing on the control board to tell you why and what is at fault. On most units if a main limit opens the burners and inducer motor will shut off, the blower will keep running until the limit has cooled down and reset.

Great I'll check that The wiring diagram does show main limit in series with the roll out switch and I know that the roll out switch (Blower door I assume) does switch off the inducer and flame together.
Is it not surprising that the upper limit would go when the duct air is only 125 deg F . The diff temp across the furnace was only 45 deg which also seems low (cold duct air was 80 de)

I have just been back to the furnace and discovered that if I pull the wire on the upper limit thermostat it does not stop the ind. Blower but just the burner which makes sense to let the ind blower cool the overheated fire box. This means that when the furnace shuts off after approx 30 minutes it is not the upper limit because the ind fan stops. Also I was wrong about the rollout switch it is not the door which is called the interlock switch on the diagram. Looking at the actual wiring it llks as if what they call the roll out switch is the thermostat next to the burners to detect flaming back in the fire box. So what is causing the furnace to cycle is still a mystery