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Failing MOT on Emissions - High Flow Cats


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Hey folks, got a tale as old as time; failing my MOT on emissions with High Flow Cats.


Taken the Z for it's first MOT under my ownership. The seller informed me the car has High flow cats and a MagnaFlow catback exhaust - I visually confirmed that it wasn't on test pipes. It's passed all of it's prior MOTs on emissions, but this time the test registers a Carbon Monoxide reading of 0.865, 4 times over the legal limit of 0.2.


Took the car out for a spirited drive, high revs, oil temp up at 110°C, and whacked it straight back into the test, but the reading didn't change outside of a margin of error. I don't imagine Catalyst cleaner will drop the result by 4x.


Outside of the test, I've regularly had the car throw a CEL for both O2 sensor banks. A prior owner had extended bungs welded for the lambda sensors to attempt to address this fault. I usually reset it with an OBDII scanner and it vanishes for a week or so.


I'm wondering whether I have the same issue experienced by this gentleman , a lambda offset applied by the ECU, causing a rich map. I'm not sure what equipment I could cheaply purchase to check for this, or whether it'd appear on a standard OBDII scanner? I don't have an EcuTek flash on this car. The car generally smells extremely rich always, not just on cold start.


Outside of this, with the Carbon Monoxide reading not changing when extremely heated (and such a high reading), is it possible the cats are completely blown through? I'm open to replacing them with a new set, but I'd hate to go through the cost and trouble only to fail again. If so, any reasonably priced HFC recommendations? I'm seeing mixed reports on the Berk Technology and Torqen ones.


The garage informed me there is no exhaust leak. I can't confirm when the HFCs were fitted, or the brand, as it wasn't the previous seller who had the work done.


Any help greatly appreciated


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Its quite possible that the car has not been remapped to support the modifications to the exhaust system and the ECU is trying to over compensate hence it smelling extremely rich. This would tie with the CEL light coming on and having to be reset as getting the car remapped removes the rear O2 sensors 


The only way I think you could confirm if you have an O2 sensor issue is by what the codes are saying. If its pre cat codes then it could be a sensor, if its post cat codes then I'd say its not been remapped. 


Depends really how far you want to go. My advise would be to find a reputable Z specialist, Abbey Motorsport, Horsham developments to name two, and get them to check the exhaust system. They'll also be able to tell you if it's been remapped. It might be worth the diagnostic charge rather then throwing hundreds at sports cats etc which won't remedy the issue at this moment 

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As has been said, take it too a z specialist and get a map check done, as new high flows are circa £600 it's got to be the cheaper and more sensible option, just out of interest on the last mot where was it done? is it local to you? , you say the previous owner didn't have them fitted so he must have got a ticket on it surely?, a remap is circa £400 and the cel light is turned of for the lambda also the car is tuned to fuel correctly for all rev ranges and I'm told drives so much smoother and responsive, high flow cats are not a plug and play item the car must be mapped to get the full advertised benefit from them, like most things Z it aint cheap but bloody hell they make you smile.

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Thanks for all of the replies guys.


Olly, what you're suggesting seems spot on with regard to the CEL - there are some proper dodgy right angle extensions, at least 30cm in length that they'd welded to alleviate the issue instead of doing it right with the remap. I believe it was a P2A00 sensor code for one of the banks, but both banks definitely threw their own code - both of them failing seems unlikely. Will get them to throw again before the prior MOT expires and drag it down to Horsham Developments to see what they have to say.


Bytespc, from reading the CO report on the 2021 MOT, it registered as 0.0, so presumably that garage is a bit 'friendlier'. Not local to me, it's in Wolverhampton and I'm in Cambridge, happy to DM the garages name to you. Hoping to get this thing done the right way though, if I can.


Given the state of the cats and exhaust system (really quite rusted, pic attached, bung visible), I wouldn't be surprised if they are just shagged though. Will report back once I've had a diagnostic done.


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Alright so, rolled the dice and ordered new Kinetix HFC's, they were recommended to me as a pair that can pass emissions, and feel like quality bits of kit.


Fitted them up at home, and realised that yep, no way in hell did my last set have a chance of passing! The Kinetix HFCs have a fine metal mesh here that continues into the cat body, whereas these are hollow the entire way through!


Have reset the ECU by leaving the battery unplugged for 5 mins, cleaned up the O2 sensors. Will post another update with the emissions page of my upcoming MOT, fingers crossed...



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