remember you are the best judge of your engines behaviour. Is it low on power using more fuel than it did.
The first step is to verify an issue then look at your engines live data especially fuel trim
a modern dash board contains a lot of information.
We recommend you take the time to get to grips with the various warning ind information lights or vehicle status
The classic method for testing the condition of your engines cylinder is via a pressure gauge screwed into the spark plug hole.
There is a better way replacing the spark plug with a pressure sensor the output of which is measured by an oscilloscope the resulting waveform reveal aspects of an engines state that the traditional compression cannot hope to match
the role of the cat is to convert various noxious gases into carbon dioxide.Todays units can outlast the life of the car
.misfires are the probably the biggest cause of them failing dont let a garage simply throw a new one on without good evidence the oxygen consumption test being a major test blocked cats are pretty rare.,
Is there such a thing as as fault free engine answer no!
All manufacturers have their quirks some more than others
Lunar auto diagnostics