Please don’t tell me that the furnace “does like early mornings”, my wife did that already. This furnace is a Carrier 58 STX 110 that has been in use (not much, I live in Los Angeles) since 2004. It run without problems until before Christmas 2017. Started by running for about 10 minutes and shutting off with code 13. I inspected for obstructions, filter etc…but because of my schedule I decided to call a Heating /AC contractor. As I expected they inspected the furnace vents, etc. and indicated that the temperature switch on the vent ducting was tripping. I verified that by checking the Resistance (Ω) value and indicated open. They checked the vent for obstruction, the air filters, vents, etc. I was at the base so I go for what the fellow told wife. They recommended to change the vent temperature sensor/switch for which they asked twenty times the cost of the switch. Since I was on duty I agreed to the repair. They replaced the sensor/switch, run it for ½ hour and declared victory. When I got home the unit did not come on at 17:00 as scheduled and it was a code 13 flashing again. We called back and they send a second guy. That day I was home so I stayed with the guy. I must admit he run a thorough and complete inspection and test. His diagnosis was a bad inducer motor. He claimed it was drawing low amperes. I not totally agree with that, I am an Engineer who works daily with electric motors, but again he was the “expert”. To replace the motor, they quoted more money that I paid for my 1965 Mustang, so I ordered the motor, plus the fan. I ordered the fan because the technician claimed that one of the blades had a hole in it. Yes, indeed it had a hole, but that hole is to permit access the Allen wrench to reach and tighten the set screw. My confidence when down fast. I ordered the parts and changed them myself, including a new gasket. The system run flawlessly all day yesterday. It was not very cold outside but we set it about 72° F and kept the house at the selected temperature by kicking on and off. At 22:00, the schedule called for 66°F. The system stopped correctly. I checked the LED solid Red, I declared victory and went to bed. This morning at 06:00 hours did not kick in as the schedule calls for. I checked the LED, code 13. I turned the main switch off and then on. The system started and followed the scheduled temperature; 72°F at 06:00, then 66° F at 10:00 hours. Please HELP!!!. I do not why when the temperature goes down below 66°F the system does not kick in. It does not make sense. From 66°F overnight to the setting of 72°F at 06:00 hours is not such a big swing. The house we had in the mountains, has sometimes, temperature swings of 15°F or more when snowing, and still worked fine. I will appreciate any help.

Looking for a second opinion. A customer called me and said their gas furnace was not working. When I arrived right away I noticed the inducer motor was not turning on. The inducer motor was getting 120 V however the fan was not turning on. I replaced the inducer motor and the furnace works for two days when the customer called and said it was not working again. I went back and there was a code on the circuit board stating the pressure switch was open when it should be closed. I bypassed the pressure switch and the unit worked fine. Therefore I assumed it was the pressure switch and replaced it as well. The furnace again worked for a day before it quit working again. The furnace goes through It sequence and turns on but right as the blower turns on or shortly there after the burner turns off. After testing about everything I could think of, the only potential issue I found is that when the blower kicks on there is a voltage drop at the inducer motor. It starts at 121V and when the blower comes on it drops to 115V and the pressure switch opens causing the burners to turn off. It is an International Comfort Product with a dual pressure switch. With the help of a friend we used a combustion analyzer to test the exhaust and the CO across the coil and everything tested fine. We came to the conclusion that it was an issue with either the circuit board or the blower motor. We changed the circuit board and still have the same issue. Waiting on the blower motor to come in but looking for any other advice from someone who may have had a similar issue.