I have a Carrier 48SS-036100331-- (about 11 years old) and my house is approx 1750sq ft. The heat works fine when the outside temp. isn't too cold (ex. greater than 50 degreesF). However, when it is colder outside, the unit frequently locks out. The exhaust fan continues to run, but the burner isn't lit. I had to have my heat exchanger replaced last season. I vaguely remember the technician saying it was likely there wasn't enough airflow, so the exchanger was getting too hot which decreased it's lifespan. This made me suspect that the limit switch was tripping. However, the led on the controller board blinks 8 times when it's locked out, which corresponds to "internal control fault" according to the schematic. I have opened all my dampers to maximize airflow. When I flip the breaker off then on again to the unit, the burner will light up again and will work for awhile and then lockout again if it is cold outside. Here are pictures:
Thanks, Casey