After exhaust fan operates for 20 sec, ignitor glows for 15-20 sec, then gas comes on for 1 sec. then goes out instantly, ignitor stays on and gas ignites in another 20 sec. This cycle continues without interruption. Consequently, heat never comes on.

Depending upon the model you may have a flame sensor down near the burners that will look somewhat like this one. Clean it well with sand paper and try to lite the burner off.