there's no "lock" as such JD, it's an 'over center' type of mechanism, once the vent is fully closed, the handle goes further ['over center'] to keep it tight - if it's stuck you need to get under there with some penetrant and hose it all good, and apply some gentle persuasion with the BFH - more penetrant, repeat

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics