I have a 2016 Octavia MkIII 1.8T petrol car.  I am getting repeated P0420 Catalyst System, Bank 1, Efficiency below threshold and 14916 rough Idle fault codes.  The fault seems to occur when the lifting the foot off the throttle when going downhill at 30-40 mph. (engine overrun).  Its hard to be precise as I can't drive and watch the engine management light at the same time.  I reset the fault codes using Carista and log the details.

 

Both oxygen sensors have been replaced recently after the garage has had two attempts at diagnosis.

 

The performance and economy of the vehicle does not seem to be affected.

 

Is it worth trying Cataclean?

 

Can anyone offer any advice please.

 

Thanks

 

John Watts

 

 

Did you ever try cataclean in the end? Did it help? 
looking at trying it myself 

First thing is putting the car on a gas analyser to see if it is a faulty catalyst.

 

Id also be checking emissions pre cat to see if they are out of wack.

 

If they are you need to start looking at why the engine is running poorly and creating a incomplete burn.

I tried Cataclean but it did not help  I ended up having a new catalytic convertor at about 50,000 miles.

 

As part of the process I moved away from servicing at my main dealer as they tried to scam me by reporting two non existent problems for attention.  The dealership is now closed.  I am now with a trusted local garage where I have received first class service.  Whilst changing the Cat my new garage found some strange error codes/settings which they dealt with via specialist support.  The car now runs like a dream.