I'm having a frustrating problem with our 2016 Monte Carlo Citigo.  It recently only just scraped through the MOT on emissions.  First it failed on high CO values, but just got through on the retest with the max permitted value after drinking a bottle of Cataclean and arriving thoroughly warmed up.    When starting off from cold, it will only accelerate to about 2000 revs, then even though my foot is buried in the carpet, it just sticks there for about 4 seconds, at 2000 revs, before it starts to accelerate once again  As soon as the engine is even slightly warm, it drives normally.   There are frequent misfires registering when cold on all 3 cylinders, but these drastically reduce when warm. but there are no codes registering at all.  It also smells strongly of petrol at the rear, but I've been unable to trace any leaks.

 

So far I've changed the following:

Battery

Spark plugs

Engine Oil

Air Filter

Both lambda sensors

 

My local garage has no idea what's going on.  Could anyone shed any light on it?

   

High CO (only) reading sounds like the cat not performing. High HC would imply fuelling is rich.

Did you fit VW/Skoda-supplied lambda sensors? "Compatible" ones often are not.

Could be a temperature sensor not 100% but not being picked up as an error. If the ECU thinks the engine is cold, it may not adjust mixture correctly.  It could also be a problem with one or more fuel injectors, or even coil-packs....