For those that need(ed) to know. Be aware that the connector for the IAT and the idle stablization valve (also shown in the diagram) can be mistakenly swapped (and the car will run like cr*p).
This G42 IAT, P/N 034 905 379B, is important to performance because the ECU needs air temps to decide about boost, fuel and timing.
In case you need to know where the IAT is and what it does, here you go:
Intake Air Temperature Sensor (G42)
The intake air temp. sensor provides another important input to the ECU which is used to adapt ignition timing and boost pressure. The UrS4/S6 ECU actually permits HIGHER boost in warm air conditions than cold air. The reason for this is to keep the engine performance consistent whatever the temperature. However, in excessively high air temperatures the ECU actively reduces boost pressure to prevent knocking.
The thermistor device within G42 has a positive temperature coefficient (PTC) which is a fancy way of saying that its resistance increases with temperature. The choice of a PTC component is no accident – should G42 fail or be disconnected then the ECU is fooled into seeing a high temperature (of approx 40 degrees C) and boost is reduced accordingly. This is a shame for those in search of an easy way to fool the ECU into permitting higher boost in cold air !
Testing the Intake Air Temperature Sensor (IAT), G42
A DMM can be used to measure the resistance of this two-pin device with the connector harness removed. With the engine OFF long enough to cool sufficiently, at an ambient air temperature of 20 degrees, a value of 450-650 ohms should be measured across G42.
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