air flow catering trailer

by steve
(suffolk)

I have a catering trailer with six LPG gas appliances when an air co2 test is done with the doors closed it failed the test.

I have had a gas value interlock fitted and 2 fans installed but still fail co2 test, my question is can the canopy door be electrical interlinked with the fans, so the door has to be open the fans have to be running then it will allow the gas value to open, is this allowed.