Use of a comma with "which" in the sentence.

[Unregistered]

Is there always a comma before which in a sentence such as - - . . . Scounted the possibilities at Countrywide Motor Sales which sells used sedans, station wagons and pickup trucks.

And . . . are restoring an old townhouse which will serve as a display shop for the . . .

I would just like to know the rule for which. Sometimes the grammer check on work says it calls for a semicolon.

Thank you.

[RonBee]

You don't always have to use a comma before which. I depends.