How is the cylinder checked for leakage?

WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.

The engine should be run to operating temperature so that the thermostat is open
The piston in the cylinder that is being checked must be set to top dead centre (TDC) at the compression phase
A metered air pressure is connected to the relevant cylinder via an adapter that is secured in the spark plug/glow plug well
A gauge is used to measure the size of the leakage from the cylinder.
This is a pressure sensor with a regulator and displays the size of the leak expressed as a percentage.

This allows the extent of any problem to be determined.