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The camshaft position sensor (G40 in the factory manuals) is located behind the cam gear and reads the 'shutter wheel', sending a hall effect pulse to the ECU to indicate when the engine is entering the compression stroke.
This signal is only used by the factory motronic ECU during startup, so a sensor that is beginning to fail will sometimes manifest as a car that fails to start seemingly for no reason, but once it starts seems to run fine, and can other times startup with no extended cranking time.
To be sure, you can test your existing sensor using this guide here.
The factory sensor when purchased from audi includes the factory bracket and costs over $100. This much less expensive alternative requires the original sensor to be drilled off of the factory bracket and this new sensor installed with the brass tabs pressed into place to secure it to the bracket.
The sensor should be wired as follows for the urs4/s6:
- sensor red wire (+5v) to vehicle red wire (pin 1)
- sensor green wire (signal) to vehicle gray/red wire (pin 2)
- sensor black wire (ground) to vehicle brown/red wire (pin 3)