Emissions fail - no cat - stock map?

[jonnyutah]

Dark blue mk5 DSG - 150k miles - he's old but has good bodywork and no electrical problems...starts in the cold too! I haven't had him long but he drives well apart from a couple of niggles.

I don't think its been mapped as it has a custom turbo-back exhaust with a decatted straight-through pipe but the engine management light comes on with a P0420 error code...however it seems to have more power than my mk7 GTi, though that could just be because it's louder 

Main thing right now is to pass MOT - mid January - and get the old guy running smooth before I worry about performance.  Basically the wastegate rattle and steering rack has started knocking slightly so those are my late spring projects, forget working on the driveway in winter!

I had the exhaust visually checked to confirm the lack of a cat and the emissions tested before I put it through the MOT and it failed big time, approx 2.5 times over the passing limit.  Would the lack of a cat cause these high readings or because the car hasn't been mapped (I assume) to use the custom exhaust system and its burning rich?

I've checked posts here and magnaflow or borla sports cats have been mentioned as best - if I get one like this, do I just have it welded to the existing straight-through pipe??

https://www.magnaflow.com/products/59959-catalytic-converter-magnaflow-standard-grade-federal-epa-compliant-universal-catalytic-converter

If I fit a sports cat, and the car hasn't been mapped, will it require mapping to limit the emissions for the MOT and to stop the engine management light flashing or would stock ECU be ok?

Alternatively as the car still has both oxygen sensors fitted, I guess it would make sense to just fit a stock cat in order to pass the MOT - but if it has been mapped and I fit a stock cat, would I need to flash the ECU back to stock for the same reasons?

If I get a cheap stock cat from europarts/ebay would that be a direct replacement for the straight-through pipe without hacking the custom exhaust as I would like to keep it (sounds good!)?  If this depends on the current exhaust system is there a way to determine what manufacturer made the exhaust, e.g. if its a milltek (wishful thinking!) I could purchase a sports cat to match.

Talking of mapping, if I do need to do this, would you recommend a DIY approach or can you recommend someone in Birmingham?

Awesome forum guys - first post, sorry its a bit boring (cats  ) but hopefully someone can help me 

[Clarkj93]

Being in Birmingham you are close to R Tech. Who ablot of people regard as one of the best tuners for these engines in the country. Lucky you, the rest of us have to drive miles to get there typically
http://r-techperformance.co.uk/

With the exhaust I'd probably give the guys at powervalve a ring. Tons of people on here recommend them. I'm sure they'd be able to find a viable solution for you.  https://www.powervalve.co.uk/index.php?route=product/category&path=308_398_546_406_411

[Jb55]

Mapping won’t fix your emissions problem for mot , pretty sure they just map out the o2 sensor when de catted so the car doesn’t throw up the engine management light.

Sports cats are hit and miss at the best of times if they’ll pass an mot or not.

Really you need a friendly mot place or put a cat on just for the mot and remove once done.

[mjmallia]

A catalytic converter is a must for an MOT..........for our age of car anyway.

[G-olf]

Hi,

Take a look at the report from the emissions test, check the lambda readings are within range, this gives you an idea if there are other problems. If its just the emissions that are out of range, co hydrocarbons etc then look at the cat, or lack of first.

The magnaflow seems to do quite well with the emissions test. The MPD downpipes are fitted with this cat I believe, there is a couple of threads on here somewhere which would confirm this.
A well looked after, healthy, cleanly mapped engine will also help as the cat will have less nasty stuff to deal with 
You will probably find at idle and light loads required during the test you will be on closed loop fuelling anyway

[LC5F]

Part of the MOT should be a visual check to see if a Cat is present... so a decat is a fail straight away.
I would say going back to standard is a backwards step, the bulk of the pre-cat harder to bolt onto the turbo and is awkward to remove if you don't have a lift.
You need to be cautious over costs to weld a cat into your existing downpipe - by the time you get get the cat there is the costs for tig welding on top - all said and done you may be better off with a sport cat downpipe and selling your current de-cat DP.
I fitted a Pipewerx one on mine, really pleased with the result, think I received it only 4 or 5 days after ordering.
It came with the Magnaflow sport cat you listed, and this provided very good MOT figures and should give years of good service:
https://www.ebay.co.uk/itm/300361586764?hash=item45eef2184c:g:BVcAAOxybi9RcZgj
For £450 they really are the best bang for buck sport cat out there.

[pudding]

If I fit a sports cat, and the car hasn't been mapped, will it require mapping to limit the emissions for the MOT and to stop the engine management light flashing or would stock ECU be ok?

Alternatively as the car still has both oxygen sensors fitted, I guess it would make sense to just fit a stock cat in order to pass the MOT - but if it has been mapped and I fit a stock cat, would I need to flash the ECU back to stock for the same reasons?

If I get a cheap stock cat from europarts/ebay would that be a direct replacement for the straight-through pipe without hacking the custom exhaust as I would like to keep it (sounds good!)?  If this depends on the current exhaust system is there a way to determine what manufacturer made the exhaust, e.g. if its a milltek (wishful thinking!) I could purchase a sports cat to match.

If it's the standard map (you need VCDS to confirm that) you don't need a remap for a sports cat.  The engine ECU just compares the frequencies of the 2 oxygen sensors and when a limit is reached, it throws the CEL. That monitoring doesn't affect the fuelling/performance.  The CEL is just a punishment from Brussels for being a naughty boy removing the cat(s).

I would personally just fit a standard downpipe (it has both cats in it) and get it through the MOT.  That then buys you a year to figure out a more long term solution.

Keep an eye on ebay for one, but expand the search to Audi A3 2.0 TFSi, Jetta GTI, etc as they all use the same downpipe.  Used ones for the GTI specifically are like hens teeth now because people just chuck them when fitting aftermarket drain pipes.

[jonnyutah]

Hey guys

Thanks for all the replies - sorry I haven't responded yet, I didn't tick the 'notify me' checkbox, and I've been so busy at work to check manually  Took a day off today though to get this cat situation sorted 

The pipewerx and tuner recommendations are definitely good to know if I eventually take that route, and the MOT tester I checked the emissions with originally mentioned picking up a standard cat - but I went with the cheaper Magnaflow and weld option (hopefully not false economy!) for getting it sorted quickly and I'm not sure what else will need replacing for the MOT.

Both nemesis - UK stockists of Magnaflow - and an exhaust specialist also reckon the sports cat shouldn't trigger the PO0420 code (assuming the oxygen sensors are working) so the car won't need remapping at this moment.

I'll let you know how the cat fitting goes, hopefully this side of Christmas - I'm using these guys https://www.blackcountryperformanceexhaust.com/ - and then the MOT in the New Year...

btw the exhaust fitters had a photo of a golf with dual exit pipes....they sure do look good

[jonnyutah]

Just a quick update incase anyone coming to this forum has similar problems - the 3" Magnaflow universal cat was fitted to my downpipe really easily.  I've been driving it for a couple of weeks now, and the PO420 code hasn't returned!

More importantly my car passed its MOT! No issues with emissions 

Only 2 advisories, front tyre is close to minimum and a front wishbone is worn but has no excessive movement