Big Sky Euro

2001 Jetta 1.8t 134,000 miles
Po420 cat below efficiency code
Decreased mileage severely 21.5mpg... Very sudden too last two tanks.
Currently idling at 16-18 on my vac gauge should be 20-22 correct?
I've had the code for a while just haven't seen any effects until now.
Safe to assume cat failure?
And if so what options do I have car will not be leaving Montana? Test pipe and o2 spacer?


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It doesn't matter if it ever leaves MT, it just can't be registered anywhere that somgs cars. Maybe that is what you meant? I mean, you cam still drive it out if state. And yes, I would figure out a way to test pipe it. Or remove cat, and hog it out, and replace, at least it would pass a visual inspection.

Yes registered is what I meant my bad. Will a test pipe, say techtronics, yield the same mileage I was getting 30-35 highway? Or would my mileage be decreased?


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Keep the cat. If anything get a high flow from TT. If you put a test pipe in without a tune you'll have a constant CEL because your O2 sensors will not read proper values. And your SAI will also throw a CEL for improper flow.

If your cat has been slowly getting plugged you'll see an instant mpg gain with a new cat.

On my S4 i gutted all 4 cats out of the downpipes, i used two sets of 18x1.5 thread pitch anti foulers to space the rear 02s out of the pipe, no CEL and awesome (for a twin turbo) mpg for 5000 miles now. may not pass a sniffer test but it shows perfect readiness in vag com and 0 codes.

Two sets, as in 4 total spacers? 2 front, 2 rear, screwed together?

You gotta post pics of that, I'm interested.

Thanks for the input.
Probably gonna go with this oem from gap.
http://www.germanautoparts.com/Volkswag ... aust/51/18

They are cheaper than I can get them through work.


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yeah, just buy two sets, youll have to drill the center to 9/16 inch so the censor will fit inside, but buy two pack of two, htey are in the HELP section at oreillys or any parts store. stack two per sensor, have done it on 3 S4's and a 2.7tt A6 and an allroad, all with perfect results, never once a cel popped up.

No spacers will go on the front o2s at all. if you want the cool guy way, google CELiminator, 034 i think has them.
Its just to take the sensor out of direct stream of exhaust and it can only "sniff' so much then.

My GTI 1.8t throws that code too. Its when I dont let it warm up. Someone has already chopped a universal cat in. Wyfee's jetta was throwing that too. Dealer updated the ECU and the issue never returned.

Or maybe cat is broken...............