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lennox furnace, model # G20Q2E-75-C1, serial #6392H32332, pilot light lights, blower motor turns on, burner doesn't burn

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Is the thermostat on? Is the thermostat getting power (or the batteries good)? Is the thermostat calling for heat? – wallyk Oct 14 '13 at 16:50
Can you hear the main gas valve open (probably a click, clank, or ping) a bit after the pilot has been lit? Can you hear gas flowing? – Tester101 Oct 14 '13 at 16:54
Does this model have an electronic pilot, or do you manually light the pilot (always burning)? – Tester101 Oct 14 '13 at 17:06

Have you followed the Troubleshooting flow chart from the user manual?

Troubleshooting flowchart
Click for larger view

You'll probably want to start with this bit...

enter image description here

Specifically, if the unit hasn't started in a bit (over the summer), I'd try cleaning the area of the burner nearest the pilot.

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Sounds like a limit switch is preventing burner ignition. Look for code flash on led light on control board.

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What makes you suspect a limit switch? How would you determine if a limit switch was not working properly? – Tester101 Oct 15 '13 at 10:42

Jump it, if it allows you, and check for continuity and check for 24V.

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