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Author Topic: Can fit a standard cat back on to a decatted car?  (Read 600 times)

Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #15 on: April 17, 2021, 12:13:37 am »
I cleared the codes but they have returned 3 times now after about 3 weeks each time. I contacted the company that remapped it and they said the codes would be because the car was mapped as having no restrictions and I have placed a restriction there (the cat) so would need to have the map tweaked at a cost of £150.

In other words, £150 to change 1 byte in the map. Downstream O2 = off, which essentially turns off the catalyst efficiency CEL. Which is illegal, being anal about it.

Bank 1 S1 is the primary sensor and the only one you need to give 2 sh1ts about. That one should never fault code.  'No signal' is a significant fault that needs investigation.

B1 S2 can do one as it's just there to monitor cat efficiency.

OEMs such as Toyota also use the downstream O2 to trim long term lambda %, but I don't belive that is the case with VW EA113 engines.

An aftermarket 100 or 200 cell secondary cat is not going to add a restriction worth worrying about.  Hybrid or big turbo conversions that are more sensitive to back flow restrictions perhaps, but not the puny OEM turbo.

Hi mate, thanks for the reply. Is it a case of just paying them in your view?

What I would really like to know is if it will damage the car in anyway. As I say it appears to be running fine and mpg seems unaffected.

Very frustrating as along with these three faults I have another 5 all relating to the of ignition switch which keep reappearing, and again no other symptoms the car is starting fine etc.  :scared:

Offline Pudding

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Re: Can fit a standard cat back on to a decatted car?
« Reply #16 on: April 17, 2021, 12:57:29 am »
No mate, your engine won't suffer any damage, don't worry.

The only thing that rings a bell of concern is the primary O2 sensor fault code.  That's the one next to the turbo and responsible for the bulk of the fuel adjustment.

It may just be a bad sensor, they only last approx. 100K miles and may need replacing, but 'no signal' is concerning.  Perhaps the wiring harness got pinched or damaged during the exhaust work?

Don't want to teach you to suck eggs, but the way it works is:

Primary O2 does all of the fuel trimming.  That's your idle and part throttle trims in VCDS if you have it.

Secondary O2 monitors catalyst efficiency and throws a CEL if the oxygen flow through the second cat is outside of expected parameters.

What most mappers do is turn off the secondary O2, which ignores the extra boost and fuelling a remap chucks down the exhaust, thereby removing the CEL and any MOT concerns.  The OEM secondary cat is enough to get through the MOT (Stock downpipe has 2 cats).

What you've done by welding in a Magnaflow cat is a good move, but unfortunately it's not big enough but I suspect the main issue is a wiring fault with your primary O2.

When I had a project VW many years ago, I welded in a massive truck sized Borla cat into the downpipe and it only just scraped through the MOT.

The long and short of it is: Aftermarket cats just do not have enough surface area, let alone Rhodium, platinum and palladium content to reliably score a pass at every MOT.   Only OEM cats meet that target.

Anyway, yeah get them to disable the secondary O2.  It's normally a standard procedure with Stage 2 and above maps, but sometimes a stage 1 needs it enabling also.

FWIW, I have Revo Stage 1 with secondary O2 enabled, stock DP with first cat deleted and it's never thrown a CEL or failed an MOT.


2007 ED30 | Ohlins Coilovers | Revo Map | Plus some other stuff | Is all a GTI needs :-)

Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #17 on: April 17, 2021, 07:22:18 am »
Thanks Pudding that's really useful info.

As an insight to my level of knowledge I didn't even know where the sensor was   :grin:

Very interesting you should mention they need replacing at around 100k as the first time the CEL came on was at 100,003 miles! I don't have VCDS only an odbeleven reader so fairly limited in what I can do.

What would your next step be? Pay someone with the correct software to investigate? Or is it worth getting under the car and checking if there is anything obvious e.g damage to the wiring, worth using some electrical contact cleaner on the sensor etc.?

From what you are saying it sounds like your instinct is that the O2 sensor fault is unrelated to the addition of the cat? But the other two faults are likely to be down to the cat. Not sure if I mentioned above I also replaced the straight pipe middle section with standard silencer.

Finally any recommendations where to buy the sensor, genuine best or Bosch?

Thanks