uPVC door won't lock: common causes
Last reviewed 2 October 2026 by the Swansea Locksmith Quotes team
The short answer
When a uPVC door won't lock, the cause is usually one of three things: the door has dropped or swelled so the hooks and rollers no longer line up with the keeps; the multipoint gearbox inside the door has failed; or the cylinder is worn. If the handle lifts but the key won't turn, suspect alignment. If the handle feels loose or floppy, the gearbox has probably failed and needs replacing.
Step by step
- Try lifting the handle firmly while gently pushing the door shut. If it then locks, the door is probably out of alignment.
- Look at the edge of the door for missing or loose screws on the strip with the locking points.
- Check whether the problem is worse in hot weather. Doors expand and catch in summer.
- Don't force the handle. A failing gearbox can lock the door shut completely.
- Measure the cylinder, or photograph the door edge, and include it in your request.
Alignment problems
Hinges and keeps can be adjusted, often in minutes, which is usually much cheaper than replacing parts. Mention whether the door drags on the frame or the threshold.
Gearbox and mechanism failures
A snapped gearbox is common on older Swansea uPVC doors. Some mechanisms are no longer made, so the locksmith may need a photo of the edge strip to source a compatible part before attending.
Coastal weather
Salt air in places like Mumbles and along the bay can corrode exposed mechanisms faster. A light lubricant on the moving parts once or twice a year helps.
When to get help
If the door will not secure at night, or is stuck shut, request quotes and mark it urgent.
General guidance only. If anyone is in danger, or you smell gas, call the emergency services first.