worcester 24i jounior non condensing approx 5years old?

boiler only makes one very short attempt to light then straight to lockout. tried changing PCB as combustion fan running all time, no joy. dissconnected purple connector from gas valve bottom left hand side, now makes 4 or 5 attempts to light before going to lock out. gas valve also now changed , made no difference. what wouldyou suggest.

jim saxton
June 2011