MK5 Golf GTI

All Things Mk5 => Modifications & Technical Area => Performance Modifications => Topic started by: WolfsburgEd on May 22, 2020, 02:50:20 pm

Title: K03 replacement DP
Post by: WolfsburgEd on May 22, 2020, 02:50:20 pm
My 2005 Gti has had aecu light on the dash for ages.

I've had the evap system changed, the lambdas changed, the recirc valve changed and am now at the point where the only thing left is the cat (slow responding according to the code).

The cat passed emissions each year on the MOT and I unplug the battery and it gives me 90 miles of free driving so can get the MOT each year!!

Now I am looking to get this properly sorted ...

I was going to just replace the exhaust with a pattern part downpipe but even that was nearly £300  :stupid:

I have seen that there are several de-cat DPs on ebay for about £100-£150 which I could then presumably get a decent cat welded in to? The two items on ebay that have 200cel cats already fitted don't guarantee that they will pass mot or state not for road use (to cover their behinds presumably). I know a guy in Banbury who could not make the DP for that price but would fit hte cat into it for a reasonable price.

Does anybody have any experience of the quality of hte basic decat DPs? Do they fit ok??

I've also discovered that I could just get the lambdas mapped out (after all of that expense on parts that could have all been fine!!) - this would be necessary with a pre-cat removal and a 200cel cat anyway yes?

Any thoughts comments appreciated.

Its kinda a last fling to get the car sorted to sell but to have some fun for the next 9 months so may even add on the St1 remap with the lambda delete mapping.

Or with Cat:

Title: Re: K03 replacement DP
Post by: LC5F on May 22, 2020, 05:21:56 pm
The problem is the cheap decat pipes can be suspect quality and many boast to be 3", but are smaller...

By the time you have sourced decat pipe, paid for a cat, paid for the cutting and welded in, the fitting + the time taken you are not far off just getting a decent UK made one that just bolts on.
I've been looking at the Pipe werx one: (

You could knock out the pre-cat, meant to be a good gain, but sounds harsh
Title: Re: K03 replacement DP
Post by: Pudding on May 22, 2020, 07:57:21 pm
Yeah just bash out the pre-cat.  P1ss easy job, but be sure to wear goggles and a face mask as you really don't want to be breathing smashed catalyst dust in.

Most aftermarket DPs are utter garbage, especially the ebay ones.  The best value quality DP is from TrackSlag, and then you get the massively overpriced stuff from BCS, APR, Milltek et al.  Honestly, don't bother.  Just bash the pre-cat out.  The gains from aftermarket DPs are minimal.  All you need to do is reduce the exhaust back pressure a bit to stop the ECU having EGT meltdowns and pulling timing.....which can be achieved by bashing one off  :happy2:

The only lambda you can map out is the second one, which is the one after the second cat and compares the signal to the first lambda sensor.  You really don't want to be mapping out the primary sensor otherwise the engine will run like a sack of hammered cr@p.