DETAILED ANSWER · UPVC DOORS AND MECHANISMS
What is a UPVC door gearbox?
Understand common symptoms across cylinders, handles and multipoint locks.
A gearbox is the central case within some multipoint door mechanisms. It transfers movement from the handle and cylinder to the locking strip. A faulty gearbox can cause symptoms such as a handle that no longer drives the locking points, but similar symptoms can come from alignment or other parts.
Diagnosis depends on the fitted mechanism. The gearbox is usually housed in the central part of the door-edge mechanism, where the spindle and cylinder connect to the locking strip. A matching replacement must suit the mechanism's dimensions, operation and locking configuration.
The same failure symptom can arise from other parts, so buying a gearbox based only on appearance is risky. A proper identification should be made before parts are selected.