Moffat Module 90 built in gas oven/grill?

Model86-01-00025. G>C. number 11-477-30.

On turning on either the oven or grill independently the electrodes arc up in both units and the oven or grill light up. However the electrode remains arcing and does not switch off until the oven/grill are switched off. Normally the arcing should stop once the unit has lit up.
However if both the oven and grill are alight at the same time both electrodes stop arcing and the oven & grill work as they should.
Could someone please advise what controls the electrode to stop arcing once the oven or grill light up.
A new spark generator has been fitted.