Gas and Oil Home Heating Furnaces - No heat on Lennox G40 unit

I have couple of years old Lennox G40 unit. It worked fine initially this year. Now when call for heat is initiated, it runs for about 5 minutes and then stops. I think I hear only the inducer fan operation nothing else.

There are 2 LEDS on the board. Out of the 2 LED's, first one if off and second is Slow flash when the furnace stops.
Description translates to:
Pressure Switch Opened or has opened 5 times during a
single call for heat, or blocked inlet/exhaust vent, or condensate line blocked(for condensing surfaces only or Pressure switch closed prior to activation of Combustion Air Blower.

What to do next? Its self ignite unit. Any help appreciated.

You need to check exactly those things it's saying may be blocked. Birds and small critters if able will get into the intake and exaust and build a home in the summer.

I've seen birds get all the way down to the inducer motor.

All vents checked. Look clean. Is there a way to find out if the Air Pressure switch is not working?

I do not see ignitor glowing. So its stopping before that.

look for holes in the tubing to the switch, water in the line, then suck on the tube leading to the pressure switch, you should hear it close, then unhook it from the switch and hook the other end back up and blow, it should be clear back into the inducer, they often get clogged with peeling paint or rust. when sucking on the switch, using an ohm meter, you should see the normally open points close. you can also check voltage going through it during operation, how many wires go to the switch?

Sound like the Induced fan needs replacing, I've had these were they just make enough draft to close the fan switch then when the fire starts the heat from the fire is enough to open the switch after about 20-30 sec. A new fan fixed it. Sometimes you can get away with bending the arm to the switch but this only a very temporary cure. The fan runner seems to deterolate and not give enough draft.

OOPs, Did you say whether the main fan started or not?? If the main fan did not start then the fire would have gone off on Hi Limit, If the fan did run then Check whether the filter is dirty or return air is blocked or there is some reason why there is not enough air flow which would cause the hilimit to kill the fire.?

I took the air switch off. When I sucked on it while having the ohmmeter connected, it was shorting properly. There were 2 wires connecting to the airswitch.

Still going with the theory that something was blocking the passage, I put a thin wire through the inlet from the fan assembly to which the air switch is connected with a rubber tube.

Put everything back together and tried. It went to the next step with ignitor glowing and the burners lighthing. So far it has worked successfully through 6 cycles.


Thanks for wonderful inputs.