So doing a bit of research and ... well a remap will obviously be the best option but the cost ...cira 550 quid + IMO is alot of money for someone in my situation (just wanting the full milltek system to work without throwing CEL codes 24/7)
So it seems it all comes down to the 4 Oxygen (O2) sensors, otherwise called Lambda sensors. Two of which are located in either exhaust pipe (one for either bank of cylinders) before the pre cats and determine the amount of oxygen present in the gas emitted by the engine as a product of combustion. In the full milltek system these O2 sensors will remain in this postion (between the exhaust manifold and cat) and so the fitting of race cats will not affect the reading. According to the Ross Tech website these first two sensors allow the engine to adjust the 'trim' settings which in effect allow the engine to 'learn' the correct mix of fuel and air within the cylinders in any given situation (idle/full load etc) to provide the optimum combustion conditions.
The second set of O2 sensors are the ones that seem to cause the hassle. In the OE exhaust system they are located between the pre cat and main cat. In the Milltek system they are located after the single 100 cell cat. From what i can work out, in the OE system they measure the amount of oxygen in the exhaust gases after they have passed through the pre cat. They don't affect the 'trim' settings and therefore dont adjust the A/F ratio. (please correct me if im wrong on this though). So their main/only purpose is to ensure that the cats are functioning correctly.
IF this is the case then the fitting of race cats (without a remap) will cause the O2 readings in these second set of O2 sensors to alter from the readings when fitted in the OE system. This disparity in readings will make the ECU 'think' that the cats are not functioning correctly (because you haven't used a Remap to 'tell' it that you've fitted race cats) and therefore thrown a CEL.
Again IF this is correct the CEL is in effect harmless as it relates to an issue that isnt actually an issue. Therefore the fitting of race cats will NOT affect the A/F ratio and cause the engine to run overly lean or rich. I know there will be other consequences caused by fitting the race cats without a remap(such as reduced back pressure) but they will only adversely affect performance slightly.
I did also read on one site that remaps such as the REVO one actually switch off the ability of a reading from these second set of O2 sensors to cause a CEL. How true this is ... i have no idea!
Now, apparently to overcome these 'harmless' CEL's you can buy an O2 spacer (sometimes people use spark plug de fowlers... i think theyre only availabe in the states though) like this one ...
http://www.awesome-gti.co.uk/product.php?xProd=32887
They mechnically distance the second set of O2 sensors from the post cat exhaust gas flow and so in essence allow the sensor to measure O2 readings similar to those seen in an OE exhaust system and therefore no CEL. A bit heath robinson but apparently they work.
Sooooo .... has anyone actually used these or have any info on the implication of their use (im sure its not as simplistic as it reads above). I did speak to a respected remapping company eariler today and they did confirm that fitting race cats without a remap would not cause the engine to run dangerously lean but didn't go into any specifics ... something to do with wideband O2 sensors??
Im i need a remap i need a remap ... ill just have to save up before fitting the cats but if there is no REAL reason to spend many pounds ATM then i'll wait till the CAF and polish and port is carried out till i get a remap
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