Volkswagen The Golf 2004 Service Training page 53

Tuning guard
In the Golf 2004, a special guard is installed for
the first time to protect the engine control unit.
The tuning guard prevents engine control unit
data from being accessed by unauthorised
persons. Any attempt to alter the permanently
stored data will be detected by the engine
control unit. If an attempt is made to alter the
data, the engine control unit will be disabled,
thereby making it impossible to start the engine.
The tuning guard is installed both on diesel and
petrol engines.
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