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Author Topic: Can fit a standard cat back on to a decatted car?  (Read 484 times)

Offline owheel

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Can fit a standard cat back on to a decatted car?
« on: April 05, 2021, 03:42:57 pm »
Hi guys, bit of a weird question.

Just under a year ago I bought a mk5 gti that turned out to be decatted. I took it back to the trader who changed the exhaust and said he sorted it out and showed me a new emissions report. I thought this was all good and forgot about it.

Unfortunately I had a pre-MOT checkup which flagged that it would fail an emissions due to the cat being chopped out and replaced with a straight pipe.

The fact that the car has been decatted would suggest that it has been remapped. If I fit a standard cat back onto the car, is this likely to cause any issues? My main goal is just to pass the MOT, I don't mind if I'm down a bit on power

Thanks in advance!

Offline Pudding

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Re: Can fit a standard cat back on to a decatted car?
« Reply #1 on: April 10, 2021, 05:35:54 pm »
So it's a stock downpipe with the second cat chopped out, and attached to that is an aftermarket exhaust?

You can attach a factory catback to pretty much anything with the right adaptor, but it won't help with the emissions if the cats are missing.


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Offline Octoparrot

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Re: Can fit a standard cat back on to a decatted car?
« Reply #2 on: April 10, 2021, 08:57:55 pm »
Yes you can, it may affect performance slightly but shouldn't cause any other problems,.

Offline owheel

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Re: Can fit a standard cat back on to a decatted car?
« Reply #3 on: April 10, 2021, 11:41:33 pm »

I've realised I wasn't very clear haha!

If I fit a standard cat back onto the car
^ I meant re install a standard downpipe and catalytic converter onto the car

The current state is that the car has a standard exhaust from the turbo back. The precat straight after the turbo on the downpipe has had the actual catayltic filter removed. The main cat has also been removed and a straight pipe has been welded in place. This effectively has decatted the car (in the cheapest way possible).

It's promising to hear that the car will not suffer from having a cat re-added though! I was wondering whether adding back pressure in the exhaust without having the map changed would cause a problem.

I'm tossing up between fitting a stock downpipe and cats, or welding in a 200 cell magnaflow sports cat


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Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #4 on: April 11, 2021, 12:54:05 am »
I had a the same problem. My car has been mapped and had a decat downpipe when I bought it so wouldn't pass an mot. I had a Magnaflow sports cat welded in which got it through the mot. About 3 weeks later I got an engine management light and following codes;



There were not other symptoms,car appears to be running fine and mpg is as expected. 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. I haven't had it done yet but Im going to book it in very soon as it's just bothering me that it might be doing damage somewhere. Also the thermostat seems to have gone so I'm not sure if this could be causing any of the codes, does seem much more likely to be the cat/map situation though.


Offline owheel

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Re: Can fit a standard cat back on to a decatted car?
« Reply #5 on: April 11, 2021, 07:27:12 am »
Thanks for your reply! Its interesting to hear you are in the same situation.

I thought adding an exhaust restriction would make it run  rich rather than lean?

The other 2 codes look like sensor failures i think? It looks like one sensor is returning a resistance/voltage out of measurement range, and the other is simply uncontactable.

I think a thermostat failure also causes the engine to run rich as it overfuels the engine to try and bring the temperature up?

Has anyone had any experience trying to flash the stock car map onto the car. Is the dealer the best way of doing this? I have no idea the quality of the map that was added, but it's not a Revo/Apr job judging by the cheapskate downpipe job, so I'm not bothered about losing it.



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Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #6 on: April 11, 2021, 07:39:28 am »
Yeah to be honest I'm quite puzzled by the codes aswell. I mean for all I know it could just be a sensor like you say, and I did read somewhere that a thermostat fault can also show up as lean so if anyone else has any thoughts on this id be keen to hear. Still leaning towards adding the cat but then wouldn't it show up sooner than 3 weeks of driving?

I tried the dealer to get the car flashed back to stock but they said they couldn't (VW Canterbury). Take it you have no way of finding out who did the map then?

Offline muff1991

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Re: Can fit a standard cat back on to a decatted car?
« Reply #7 on: April 11, 2021, 07:41:08 am »
If you’re after a cheap stock cat (used of course) I might be able to hook you up, it passed the mot in January, I have just fitted a decat. I got a stock map put onto my car by a local garage who also does generic mapping, he only charged me £50.


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Offline owheel

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Re: Can fit a standard cat back on to a decatted car?
« Reply #8 on: April 11, 2021, 07:46:29 am »
@Muff1991 I am definitely interested in your cat. Where are you based. I'm also interested in what your plan is for future Mots? Are you going to swap a cat on to the car each time?

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Offline muff1991

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Re: Can fit a standard cat back on to a decatted car?
« Reply #9 on: April 11, 2021, 07:49:10 am »
@Muff1991 I am definitely interested in your cat. Where are you based. I'm also interested in what your plan is for future Mots? Are you going to swap a cat on to the car each time?

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I’m based in Aylesbury (bucks) and I know a guy who will help me through the MOT with lending me a cat


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Offline owheel

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Re: Can fit a standard cat back on to a decatted car?
« Reply #10 on: April 11, 2021, 05:23:16 pm »
@Jons1001 your codes are really puzzling! Are the lower 2 sensor codes in the picture both relating to bank 1 sensor 2?

The first code means your engine seems to think it's running lean from the data from the sensor upstream of the cat. If both the other codes relate to bank 1 sensor 2, then it looks like that sensor is broken?

Did these throw a CEL on the dashboard?

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Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #11 on: April 11, 2021, 05:53:25 pm »
@Jons1001 your codes are really puzzling! Are the lower 2 sensor codes in the picture both relating to bank 1 sensor 2?

The first code means your engine seems to think it's running lean from the data from the sensor upstream of the cat. If both the other codes relate to bank 1 sensor 2, then it looks like that sensor is broken?

Did these throw a CEL on the dashboard?

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Hi mate, tell me about it!




As you can see the bottom one is bank 1 sensor 1. Yes the CEL did come on and has done every 3 weeks or so since the fault occurred a couple of months ago. Really need to get to the bottom of it so if anyone has any ideas would be great.

Offline owheel

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Re: Can fit a standard cat back on to a decatted car?
« Reply #12 on: April 12, 2021, 04:27:00 pm »
@Jons1001 Do you think it could be a damaged o2 connector or loom if it's an intermittent problem? Maybe it could be that every 3 weeks or so you are hitting a bump just right that is momentarily breaking the connections/causing funny readings? Those codes would recorded but would not affect driving. It would allow you to clear them but they would reappear as soon as the loom gets jogged again. All three codes are related to the O2 sensors, so you have one code for sensor 1 Saying the engine is running lean, and another saying the same sensor is stuck rich and cannot be reached? If the sensor can't be reached, then surely it cannot tell if the engine is running lean or not?

My logic is that if these cause a CEL and you can clear it with out physically changing anything on the car, and it doesn't re appear immediately then the car is not actually running lean and the exhaust is not rich as per the codes, meaning it must be an intermit sensor/loom issue.

Can you read live data with your app? Maybe run the car and see if you can read data from the O2 sensors and see if you are getting signals?

I want to stress though that I am not a professional mechanic, I'm just using my experience from working with sensors at work. If anyone knows more than me I am happy to be educated!

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Offline Jons1001

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Re: Can fit a standard cat back on to a decatted car?
« Reply #13 on: April 12, 2021, 07:30:27 pm »
Yes interesting thoughts. Im having the thermostat done next week so will ask the mechanic to take a look and see if he can see anything untoward with the O2 sensor/connectors/wiring. Not sure if i can access the live data, I have a basic version obdeleven and while it does have live data on the menu it looks too technical to use for someone like me.

I take it though that it could also be to do with the remap as if they say it was mapped to have no restrictions and I have now placed a restriction there this could cause these codes?
« Last Edit: April 12, 2021, 07:33:19 pm by Jons1001 »

Offline Pudding

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Re: Can fit a standard cat back on to a decatted car?
« Reply #14 on: April 16, 2021, 11:56:04 pm »
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.





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