UK Legacy failed emissions test

[Neil]

Help !!!!

Uk spec 2.0 turbo 4 cam 1993
Failed Uk Mot test due to emmisions off the scale !!!!!
Air and oil filters have been changed as have the plugs ( not an easy job )
Anyone got any ideas anyone ??

Ps occasionally smokes when started but runs and performs ok.

 

[Neil]

Update !!!
Have now found out that the Oxygen sensor is to blame for the high Lambda
readings and have tried a Bosch universal part as a replacement but this has
the wrong connections so its gonna need an expensive Scooby part .
Does anyone know of a source of good quality Scooby bits in the UK that
might beat Subaru themselves ??
I live in South Northants area but will travel to get the right parts.
Big thank you to all who have tried to help so far - ive given it a dose of
additive and had already tried the hooligan bit Marcus (good fun as its a
quick beast when you poke it with a stick !!!) but its not worked yet.
It does have a Cat but all who have looked at it say its ok.
Neil