Another "16804 efficiency below threshold" question!
I've got the dreaded 16804 "efficiency below threshold" fault code (which seems to be not too uncommon from the searches I've done). In my case, the car is a 2010 S3 and is (or should be, it had one previous owner) totally standard, no remap etc.. I've been clearing the code in VCDS to see how long it takes to reappear and it's usually popping the "check engine" light back on within a few days.
It sailed through its MOT emissions test and the fault had been showing for at least a month or two before that but I had my friendly garage run diags on the O2 sensor and the cat anyway. They said the O2 sensors checked out fine but a test if the cat showed that it wasn't effective enough to keep the O2 B1 P2 (bank 1, post 2 I believe) stable.
Now, that MOT was it's 3rd birthday MOT. Should a cat be failing after just 3 years? I've been quoted £1200 all in to have it replaced with a genuine Audi cat. It doesn't make any difference to the way the car drives, so I could just keep clearing the fault and ignoring it until either I can afford the replacement or the emissions cause it to fail an MOT. I don't like driving around with the orange light on, partly because it's annoying to see but mostly because any other more serious fault would be masked by the light already being on.
Any advice? Bite the bullet and get it replaced or is there any other alternative? The chaps at my friendly garage said that, in their opinion, they wouldn't recommend aftermarket cheaper cats because they've seen many of them fail again not too long after installation and that it ends up being a false economy. I'm not sure if that matches with your experience.