epic MOT emissions fail... lambda?

[cvega]

I'm not really familiar on causes for this. Car runs perfectly (996.2, 3.6), rebuilt 3k miles ago, included new cats and exhaust backboxes. No CEL, no hesitation, no symptoms... apart from MPG's. struggling to get past 25mpg.

Especially the 2nd fast idle test, lambda reading is off the charts. Am I guessing right one of the lambdas may be on the way out ?

How can the two attempts be so different? I mean, every reading is not even close to attempt 1..

Edited by cvega on Tuesday 10th October 09:16

[Sir Loin]

What cats do you now have?

[cvega]

Topgear 200 cell ones. Car was warm (took it down for a blast on the Mway).

[Sir Loin]

Cats are most likely causing the readings. Mate has a rebuilt C4s with 200 cel cats and had the same problem. Just find a friendly MOT place.

[Slippydiff]

Or fit some O2 sensor extensions :

https://www.design911.co.uk/porsche/996--911--1997...

[cvega]

okay, a bit surprised as Topgear states they are direct replacements for OE cats.

[BAMoFo]

It could be an air leak that is causing the engine to run very lean at fast idle.

[MarkM3Evoplus]

due, often to bore size of the exhaust and how far the lambda probe goes into a 911 exhaust, air get sucked back and causes a fail.

if the tester will block the tailpipes in the exhaust with rags, it will pass

[bevsy]

Maybe pulling air somewhere? Or as stated above block up tail pipe if tester will and lamba reading will come right down

[gtsralph]

bevsy said:

Maybe pulling air somewhere? Or as stated above block up tail pipe if tester will and lamba reading will come right down

The 200 Cell HJS cats on the Cargraphics manifolds fitted to my 3.8L Cayman S failed twice first after 7k miles then after another 20k miles. They tend to break away at the edge of the cylinder as pictured. HJS provide exhaust manufacturers with a two year warranty from date of HJS manufacture. Luckily my failures were within this time limit else £500 per side.