Hi to all, economy my my 1.8 t vrs, mark 1 has dropped from 35mpg to around 25mpg. No eml came on but car feels a bit louder on start up for first 2 minutes. On scanning with vcds lite the fault code 17584- bank 1, O2 ( lamba) correction behind catalyst ,control limit reached p1176-35-00- came up. Is it as simple as changing lamba sensor behind cat or does anyone have any other ideas? Is it ok to leave as eml light has not come on for a few days? Thanks

The loudness on startup is the secondary air injection pump. the rivets on them fail and they start to split apart and leak. All it does is pump air into the cat for a couple of minutes to reduce cold start emissions and it's not essential.

As for the other - check all of the turbo and vacuum pipes for leaks. I had a similar error when I had a boost leak.

Though it might be the lambda sensor. They're not that expensive.

Check pipes for leaks first though, that's the most likely thing, pretty common for them to split when the car gets to a few years old.

This is what Ross-Tech's Wiki says for that fault code:

17584/P1176/004470 - Bank1: O2 (Lambda) Correction Behind Catalyst: Control Limit Reached

Possible Causes

Intake Air System leaking

Oxygen Sensor(s) and/or Oxygen Sensor Control faulty

Possible Solutions

Check Intake Air System for Leaks (False Air)

Check Oxygen Sensor(s) and/or Oxygen Sensor Control (all of them!)

Thanks guys, will check for leaks before moving on to sensor. Many thanks