what can go wrong with a throttle body? (NAC)

[Douglas Frank]

This is about a New Beetle, and I'm asking here because over on vwvortex 
they're busy discussing stick-on vinyl flowers and what to name your 
car... good thing the world's as large as it is... I never had much time 
for the flower children 30 years back, either... 'nuff said.  :-)

Anyhow I've got a fault code on a 2L engine: 17990, or in plain English, 
"Idle Adaption Limit Reached."  Oh!

Took it to the dealer, and he's telling me I need a new:

1. MAF sensor

2. throttle body

Now this would be the car's 3rd MAF, it was replaced about 18 months 
ago.  At least it's still under an extended warranty.  But my real 
question is, what that heck can go wrong with a throttle body that would 
require replacing it?  ...other than hitting it with a sledge, that 
is...  Whatever he's smokin', I want some too!

BTW they did claim to find and fix a cracked tube in the fuel rail-- 
funny, I never smelled gas under the hood.  It's a '00 GL, btw, with 
~27K on the clock.
-- 
Douglas Frank            Incumbent, n.
HP Co.                     A person of the liveliest interest
110 Spit Brook Rd.         to the outcumbents.
Nashua, NH 03062 USA