Heres my experience, apologies if its slightly long and comprehensive in detail...
I've got a 1999 1.4 8v CL with MOTRONIC MP 9.0 ECU - Type 6N1
if it was a GTI it would more than likely be a 6N2 with Marelli ECU if I am not mistaken?
The scan tools used by some garages (e.g. Bosch KTS 200 used by Kwik Fit) .... with dual meanings or vague descriptions, making you look in the more expensive direction instead of the simple things that may cause components to fail (so sometimes deeper the diagnosis, the better)
VAG codes are generally more conclusive than OBD codes, personally speaking from experience... because the VAG group wanted to be special, we all know they are
-------------------Problems I've had------------
Started with a misfire on cylinder 4, causing catalytic converter to overheat and fuel consumption to sky-rocket
got OBD codes of P0215 --- idle control above adaption limit (could also mean... FUEL INJECTION PUMP CONTROL CIRCUIT)
and P0231 --- Lambda adaption additive too low
got a VAG code for them 00533 - Idle Speed Regulation Adaptation Limit Exceeded (additive too low)
Something the codes had in common was a faulty fuel pump relay.... turns out not so, so far! how confusing!
-------------Solutions--------
Replaced the sparks then distributor cap and HT leads, cured the misfire... for now at least...
Cured the fuel consumption, CALMED the erratic idle and stopped the stalling during braking/gear changing, for now... by:
Adding some Redex carb cleaner... I think this should be a mandatory part of the driving test... particularly with Polo owners... erratic idles/fuel consumption/stalling appears to be a common one!
reseated the air filter on the throttle body and the tube - so far, no more P0231 codes or 'additive too low' errors.
------Prognosis----
The catalytic converter needs replacing now anyway....
still getting the P0215 on the bosch - this could just be a throttle body adaption calibration as the battery was replaced by monkeys? which the bosch KTS 200 cant do!!!
I'd like to inspect the Idle Air Control Unit first and the throttle body re-calibration before going for the whole throttle body replacement...
Note:the following requires a certain level of common sense/self-restraint, problem solving and may require the process of elimination in some instances and support from people in the know-how... and generally a model made AFTER march '99, when they implemented the crypto for their ECU's
The Autel Maxiscan VAG405 (around £50) appears useful for VW initial diagnosis - shows VAG and OBD codes, but relys upon the accuracy of the sensors but is UNABLE to do basic settings or adaptations, e.g. Throttle body adaptation!
For such tool, maybe a Memoscan VAG5053 (around £80) would be best suited, to someone who knows what they are doing with basic settings and adaptations...
The true geeks tool for geeks - VAGcom(now VCDS) is more useful for seeing the absolute raw output of sensors, may perform more functions, and will warn you better (£200+)
I have discovered that vehicles with the Bosch Motronic MP 9 ECU, may suffer from the 00530 or 00533 VAG code bug, which
"may" also translate to P0215 in OBD codes
Basicly, whenever that has a problem, Get VCDS first to diagnose, and only if you know what you are doing, as a last resort, run 'ResetMP9' from:
http://www.hexdiagnostics.co.za/downloads/resetmp9
So basicly the ECU can give you the run around
Hope this helps some... again apologies for it being long and in depth!
I take no responsibility for anyones actions resulting from reading my post...
Koolie.