I recently changed the flame sensor on my Comfortmaker furnace, because the flame would come on and burn for only a few seconds. However, this did not solve the problem. Is it most likely the gas valve?
For how many seconds do you have flame?
Do you know where your pressure switch is located?
Ok. Look for the small fan motor in the burner area. There will be a rubber hose coming from it to the pressure switch. Remove the rubber hose and check the port on the inducer motor and see if it is clear and free of any debris. You can put the hose back onto the port and see if you can blow into it. After that put the hose back on and give another call for heat.
Take a look at this youtube video.
What if I don't have an OHM meter?
You do not need an OHM meter. I just wanted you to know where the pressure switch is. Do you see it?
Find the rubber hose that runs off of it and follow it over to the port that is on the inducer motor housing. Take it off there and with a small paper clip see if it is free and clear of any debris. Then put the hose back on and see if you can blow into that port (not the pressure switch). After that is done put hose back on and see if you can get a call for heat.
No obstruction, but it still does the same thing.
Ok. Next step is to shut the power down and then remove the two wires going to the pressure switch. Then give a call for heat and when the inducer motor (small fan motor) comes on you will join these two wires together. You can use a small piece of thermostat wire, or even a paper clip to join them at the spade terminals. The important thing to remember is to join them together after the inducer motor has come on and not before.
That did not change anything.
Lets check the high temp limit switches, and the rollout switches.
You will be looking for something similar to the following pics.
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The last one will only have the plate and the two connections showing with two wires going to it.
Once you locate these, with the power off, press the small button on top of each one, if there is a button.
Let me know what you find and what happens after you do this, and try to start the unit back up again.
Do you see the furnace wiring diagram anywhere?
Ok, so they are automatic reset instead of manual. That explains why it will attempt to restart repeatedly.
Can you find the high temp limit switch? It should be located so that it detects the temperature of the heat exchanger compartment. They are usually mounted on the "firewall" or on the side of the housing that contains the heat exchanger.
Once you locate that, place a jumper on it, to bypass it temporarily.
Let me know what happens then.
The flame stays on with the jumper on. Does this mean I need to definitely replace the fan limit switch?
That is correct. The high temp limit has tripped, replace it, and make sure that the airflow through the heat exchanger and exhaust is free of any obstructions.
It would be a good idea to clean the burners and igniter assembly at this time as well.