This replaces the stock ceramic cat which can disintegrate. If your cat is making a rattling noise then replace it with a stainless steel higher flow cat (200 cell) and see a quicker throttle response and a slight change in the exhaust note.
When MOT is due they will warm up and cool down quickly so make sure your tester has a warm cat on arrival. Some cars require a MIL eliminator. This is wired into the sensor and prevents the MIL light from triggering. (Not all cars need this)
The longer cat was fitted to some later MGF's (approx 1999 onwards) and all TF cars.
The best way to identify if your MGF has the longer or shorter cat is to look under the car .
Please check that your country will accept these as a replacement for the stock cat..eg TUV requirements.
Note:
If you car has a aftermarket back box then it may be fitted with a link pipe to connect to an earlier shorter cat. This Mk2 cat allows you to connect directly to a back box with the 90deg outlet.