Just to add to the angst from t'other thread on another forum....
My Mazda 6 T2 'pickup truck' developed a vibration I could feel through the wheel and the soles of my feet. I took it to my very local garage, where the youngish guys gathered to have a peek and poke inside. "It's the cat converter, mate. That won't be cheap. Main dealer part - I can get one here for Friday."
I chose to 'think about it', and yes, that replacement would be over £1000, with one thing and another.
So I dropped in to a Mazda main dealer - well, two of them, actually, in Bristol and Trowbridge. The Bath Main Dealer was 'too busy' again - asking that the issue be diagnosed properly. I told them I'd asked a local garage, they'd guessed 'cat converter' with fallen-off bits that rattle about inside, and that I was unconvinced. Said car disappeared into workshop(s) and eventually reappeared. "It's the cat converter, mate. That won't be cheap...". Twice.
I'd already determined that a faulty cat converter would show up at MOT time, it wasn't really needed in the interim, and a used one could be had for £250-300 via the interweb. Fitting would be an hour's billing. I did want to discover which Mazda main dealer I could trust, or not. I got my answer....
I wandered away, displeased with misfortune and my share in it.
Some weeks later, the vibration increased significantly then, passing fast along the dual carriageway north of Plymouth with friend Sara in the co-pilot's seat, there was a loud bang and serious rattling, with multiple panel warning lights. I stopped pdq, as one does. It emerged that the alternator had destroyed its bearing(s) and had failed catastrophically. The vibration had, in the words of the RAC patrolman, been a warning I hadn't heeded. OK, so a new alternator went in, for ~£300. End of vibration.
And the vehicle's exhaust system - including catalyser - in the latest MOT examination, passed clean as a whistle.