Johnson Starley JB25-30 burner fires up but only runs for seconds?

Hi
Any advice would be appreciated, we have a Johnson & Starley JB25-30 Modairflow series 2 system. The problem is that the gas burner fires up but only stays running for a couple of seconds, the pilot light is staying on and has not gone out at all.
We have replaced the Overheat Limit Stat and air flow sensor. We have also tried bypassing the room themostat. The fan runs fine when switched to continuous. We are wondering if it may be the control panel, any ideas?
Thanks