Dryer works with the vent off, but not when it’s connected. What next?

That difference points to an exhaust airflow problem, though it doesn’t prove the duct itself is the only cause. Check the outside vent while the dryer is connected and running normally. The flap should open freely and there should be a clear, steady flow of air. A stuck flap, blocked outlet, crushed hose, or a sag that collects lint or water can still restrict airflow after brushing the duct.

Don’t use a gas dryer with the vent disconnected. Exhausting indoors can put combustion gases and moisture into the living space. If the outside outlet is hard to reach, have it checked safely rather than climbing onto a roof.

A clean duct can still be too restrictive. Check the dryer manual for the permitted duct length and fittings, then compare that with the actual route, including bends. Make sure the hose behind the dryer isn’t pinched when the machine is pushed back, and use the proper rigid or semi-rigid duct type for the installation. A long, bend-heavy run may need a vent professional to assess.

Also check that the exterior outlet isn’t covered by a screen that traps lint. The outlet should open freely and meet local requirements.

If the vent route and outside flap are clear, the dryer itself may not be moving enough air. A damaged or slipping blower wheel, a blockage in the internal air path, or another airflow fault can show up more when the dryer has to push through the full duct. Cleaning the lint filter and visible lint won’t rule those out.

For a gas model, leave internal repairs and any fuel setup questions to a qualified appliance technician. If it has unusual heating behavior or you smell gas, stop using it and arrange service.