I have an '03 315CL that is equiped with a pattern changer. It is mounted behind the cab, and must be accessed by openning the driver side access panel. Look for a block that has pilot hoses attached to it, and it will have a lever (about 2" long) with a hole in the end, that will accept a set screw (it's a bolt actually, with a 1/2" head). As was described above the screw must be removed, then the lever can be set to the other pattern (or patterns-I've heard). The lever has some resistence so, in my opinion, the set screw is not mandatory. I don't see it changing positions on it's own. I think that the majority of Cats are ordered from the factory, by the dealers, with this option so it has become fairly common. If you have a manual it should tell you where to find the pattern changer. If it is a 312 (not a "B", "C", or "D") the pattern changers were a lot less common back then, so the chances of yours having one will be slim.