Screw cap on the cat

[Guest Roger Jones]

VR6 Highline, December 1996, 42k

I've just replaced the screw cap (19mm hex nut) at the back of the cat on the offside. The old one had disintegrated and exhaust fumes were therefore venting, which would have been an MOT failure item. The cat is perfectly sound, but the screw cap must be made of inferior stuff.

Why is it there? Does something else screw into that hole on cars destined for non-UK markets? Or is it simply a hole for a testing/measurement device?

Does anyone know the correct torque setting for it?

By the way, Wayside parts dept (St Albans) couldn't find the cap on their exploded diagrams and a whole-cat replacement nightmare started stirring in my mind. However, my mechanic managed to source it via M. J. Warner at Hare Street, nr Buntingford, Herts.

Part numbers:

* screw cap for cat converter 4A0 131 737 A -- "cap nut"

* washer N 013 830 2 -- "seal"

[purple highline monster 0]

lol, i had exactly the same problem mate, the bolt disintergrates due to the heat/wet/cold as its only a steel bolt, i just had a mate weld mine over in june and its been fine! i cant really see its anything than an inspection port or something? you can get the bolt from vw its listed in an obscure place if i recall, welding it up has been fine though!!

[Eat this 2]

its not a lambda sensor is it??

[petervr6 0]

na not a lambda sensor purple highline monster says prob for inspection purpose only had to weld mine up last week.

[VR6Pete 1,455]

same happened to mine.

[purple highline monster 0]

i think its common thing on most cats, i remember when i worked for VW and ordered new cats for workshop etc i always forgot to get the washer and bolt!!

[Guest Roger Jones]

On the Honest John forum I've learned from Andrew Moorey of Tune-Up Ltd (Horsham) that the port is for a secondary lambda (oxygen) sensor. Someone else has suggested that it is used in some countries for routine checking that the cat is working properly; maybe all VAG servicing garages use it at major services?

[purple highline monster 0]

i would of thought it was too far down the system to be effective for a lambda sensor? could be a US thing i suppose their emmision laws are stupid! as for VAG you would like to think but i doubt any tech at vw checks a kat that thoroughly on a service unless its hanging off! hope not anyway as mines welded up!!

[Guest Roger Jones]

Mr Moorey really knows his stuff: if I recall correctly, he has tuned/raced VR6 Golfs in the past. I think the port is to allow tests of exhaust gases having passed through the cat, which makes sense if you want to find out if it's working properly. Hence its secondary status.

[purple highline monster 0]

ah that makes sense then, mines still welded up though!