BS4

What does it do? Is it necessary to reset your ECU with this to prevent dealer from knowing car is chipped, or can you just replace stock chip? Also, if ECU "learns" new mods, higher octane, etc, why reset it? Just wondering how necessary this step is.

Just a stock S4

The VAG tool's real value is as a general diagnostic tool. It allows you to read the fault memory in the car - if something reports an error (check engine), you can read the cause.

Also rest things like lock programming, etc.

Grant

Bichon

There are three kinds of ECU resets:

1. Reset fault codes. For instance, with a VAG tool you can erase the power loss fault code that is generated when you swap ECUs.

2. Reset throttle adaptation. You can use the VAG tool to trigger and monitor a throttle adaptation. This allows a new ECU to learn the resistances associated with various positions of the throttle plate position sensor, so that the ECU can manage the throttle better. (You can also attempt to trigger a throttle adaptation by turning the key on for three minutes without starting the car, but sometimes this doesn't work)

3. You can reset the timing adaptation data, not with a VAG tool, but by unplugging the ECU, or by disconnecting the negative battery cable for about 15 seconds. The reason for doing this is knock control on the S4 is very conservative, and pinging caused by substandard gas or other reasons will cause retarded timing for an extended period of time. Reseting the ECU after refueling allows the car to regain full power immediately.

BS4

will it do things the ECU can't do without resetting? (I understand erasing the power off fault code)

Dr J

where can you get a vag tool, and for how much(do I need laptop?)

Just a stock S4

The real thing is about $1500 (??)
The beta versionof the emulator, for a laptop, with adaptor is $200 from ross-tech.com

Will work from some desktops, caveats from ross (well, uwe, actually). nice guy.
Grant