NO IGNITION FROM GAS FURNACE

[DIYRBM]

TRANE GAS PACKAGE UNIT W/ ROBERTSHAW SP715A INGNITION CONTROL UNIT NOT PROVIDING IGNITION SPARK. CALL FOR HEAT, VENTOR MOTOR STARTS, PILOT VALVE OPENS BUT NO SPARK. CAN ANYONE PROVIDE IDEAS FOR ISOLATING PROBLEM? I SUSPECT CONTROL UNIT. IT HAS 24V POWER AND IF YOU LIGHT PILOT BY HAND MAIN BURNER WILL LIGHT.

THANKS IN ADVANCE