Hi Simon

Replacing both O2 sensors as a matter of course, sounds a bit over the top to me.

I know that we shouldn't be over concerned with costs, but rather make sure that the repair is correct, but often a quality catalyst will cost around 400 quid & then there is the fitting.

If we were to replace the O2 sensors routinely too, then the car will most likely be written off.

Where do we stop? Oil & filters, spark plugs, MAF sensor etc etc?

Having said all that, we do need to be sure that there isnt a fault elsewhere. I wouldn't like to spoil a nice new catalyst, by over looking an underlying problem.

Better to test & identify the cause I think!

This is a deep topic isnt it!