Thanks for your reply.
Well, the furnace kicks in when the switch for heat is turned down to "off" then "auto".
I opened the furnace cover but did not notice any LEDs blinking.
Can you tell me where to look at?
We had similar problem before and the lennox technician replaced the johnson control board and it worked for three years.
I double checked it. I don't see any LEDs blinking. Is it red colored?
by the way, the furnace is 15 years old and the relay controller is "Johnson Control" with model number 63K2401.
Okay maybe this furnace doesn't have a diagnostic led on the board. Is this a standing pilot if so is the pilot lit? What does the furnace do on a call for heat pilot lights but burner never comes on or the burners come on for a second and then shut off?
The pilot is lit all the time.
Again, the furnace starts when the switch in thermostat is turned to "off" position and back to "auto" position.
So it seems that burner works.
The problem is probably the high limit tripping or the board like you said before you can verify the high limit is okay if you use an electric meter and ohm the high limit out if it ohms out okay then it is probably good but you will have to check it when the furnace will not work as it is an auto reset so when you turn the thermostat to off and then back on it resets and the furnace will run a cycle till the limit trips again then you will have to reset again. You can also unbend a paper clip pull the wires off the high limit and use the paper clip to jump the wires together and see if the furnace will run more then one cycle if it does the high limit is bad. I am attaching a picture of the limit switch it is usually above the burners in the center area of the furnace.
Thanks for your note and sorry for my late reply.
(1) The resistance is about 8 ohms from the thermo-resistor, 18J3701-the limit controller. So the limit controller seems OK.
(2) The fan of the furnace started when the wire to the limit controller was pulled out. The fan stopped when the wires were plugged back in.
(3) When the wire is pulled out of the limiter, the fan did not stop even when the switch is turned to "off" position.
(4) I found there is green LED in Johnson controller. But I never see it blinking any time.
Is there any way to make the LED blinking?