Some of my blogs are of the “I’d better write this down so I don’t forget it” variety. This is one of them. This is intended primarily for future me, but it would be cool if anyone else finds it useful.
The equipment in question is an American Standard (Trane) Freedom 90 gas furnace that’s about 7 years old. Recently, in fact right in the middle of a record-setting snowstorm, it started blowing only cold air.
I followed the instructions in the manual to reset it (off-on-off-on in less than 30 seconds), to no avail. The burners would get ignited, then seemed to flame out. The control board then tried to restart the process. This would go on 5 times or so. After that the blower fan would be going, but only ice-cold air was coming out. The red light on the control board flashed twice in roughly 3-second intervals. According to the manual, this indicated an “External Lockout (retries or recycles exceeded)” error.
Some online forums (google “external lockout”) implicated the flame sensor. Apparently it needs to be cleaned about once a year. I’ve never seen it much less touched it, so it’s highly suspect. Before working inside the furnace, it goes without saying that you must first turn down the thermostat and shut off power to the furnace. To get to it, remove the two panels on the front of the furnace starting with the bottom panel.
The burner unit is in an enclosed box at the top of the cabinet. There’s a round viewport on the front bezel where you can observe the burners in action (provided they are running).
Remove the front door with an 8mm wrench. It hinges at the top and can be tricky to remove.
Locate the flame sensor.
Remove it with a socket screwdriver.
Thoroughly clean the sensor’s rod and contact with sandpaper. I’d use 150-grit as well as a finer sandpaper.
Replace the flame sensor. To test it out, you should replace just the burner door and the bottom panel of the cabinet; there’s a door switch that prevents operation unless the panel is in place (or it is manually depressed). You can view the burners through the viewport. Turn on the power and turn up the thermostat to call for heat.
In my case, the first attempt wasn’t successful. Before calling for service, I decided clean the flame sensor again even more thoroughly. There was some rust on the contact that I made sure to remove with a fine-grit sandpaper. This must have done the trick.
Note to self: Clean the air filter while you’re at it