The unit started with a 33 code. I resolved that issue but not the furnace runs constantly and will only heat up to 67 degrees in the house. All appears to be working when I look at the furnace. Flame is on and blower is running. The furnace does not cycle so it is not over heating. Air from heat ducts is warm but not hot enough.

33 on many units is the pressure switch. Since the unit appears to be working, I would check for a disconnected duct, insulation off the ductwork or something similar. I just made a service call and found that rodents had torn off and chewed open 5 ft. of ductwork in the attic. Depending on ambient temps, high wind chills, leaky windows, poor insulation etc, you may have reached the output capacity, particularly in an older house.