Ignition failure
No gas pressure.
Gas pressure under minimum value shown on rating plate.
Gas line not completely purged of air.
Defective Electrode.
Loose burner ground connection.
Defective Ignition Cable.
Defective gas valve (check for 24 Vac at harness during trial for ignition before replacing valve).
Air-fuel mixture out of adjustment - consult factory.
Defective or fouled electrode.
Plugged drain line in trap.