Audi Repair: Fluctuating Temp Gauge A8, engine coolant, engine compartment

Question
My 2000 A8 with 125k miles has a new water pump, t-stat, and belts yet the temp gauage continues to fluctuate between cold an normal. Could it be the ECT sensor? If so, where is it located in the engine compartment and is it easy to replace?

Answer
The Engine Coolant Sensor would be my bet too; it is a fairly high failure item.  Usually located in the cylinder head or close to the thermostat housing.  not sure on the A8.  Usually it is a straightforward replacement, you may lose a little water so do this when the engine is cold or lukewarm.  Let me know,  Jan