My unit works normal unless the temp outside drops to the low teens. At night our programmable thermostat is set to 65 for sleeping. In the morning it is set to 69 for the day. If the evening temps only drop to the 30’s the system will warm up to the set temp of 69 in a very reasonable time (30 minutes) and maintains it all day with no issues, but if the temp drops to the teens or lower, five hours after calling for 69 the temp will have only raised about 2 degrees to 67. The system runs and cycles on and off every 10 -15 minutes as if it’s overheating. The airflow coming from the floor vents is light and not very hot. Hears the kicker, if the top cover of the unit is removed to see the LED and left off, the air flow through the vents increase significantly and is much warmer, and the house will reach the set temp in about 20 minutes. The filters are new and changed every three months. The thermostat is new. I removed and cleaned the flame sensor, and removed and inspected the hose for the blocked vent and safe guard switch. Any ideas as to why the system acts as if the air flow is restricted. Thanks for any help and suggestions.

Sounds like you have a restriction in the return air. You say your removing top door too see the led light. I assume it's a down flow system. Is it LP gas or Natural gas?