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number one cause of fault is dirty flame sensor. Locate metal rod in front of burner with white ceramic base. Clean rod with steel wool. Get back with me with the results.
unplug hot surface ignitor from molex connector ...set voltmeter to ohms. Real handy if you have the tone setting so meter beeps when leads touched together.
Then put meter leads on prongs going to ignitor where you unplugged it. Open means bad ignitor.
This is a common part available at most every heating contractor shop and available on line.
here is example - http://www.climatedoctors.com/items/item.aspx?itemid=772994
Part number 1474-052P
Removes from burner section...very easy to replace.