The only thing that puzzle me is that the aux fan is an electric fan, and all people I've talked to so far told me electric fan has no clutch. It either runs, or it doesn't.
On my old car (W124) , the fan clutch wear out once, and was replaced. But that was the main one, and fan clutch was test be rotating the fan, and saw that it spin freely.
However, it's the aux fan that have problem now.
Originally posted by Meza
Two things you may want to do:
1. Flush your cooling system
2. Check your fan clutch.
3. If the fan runs, then it's ok and doesn't need replacement like the mechanic said. Those fan last a long time without problems.
4. The fan should run at low speed the minutes you turn the A/C on. When the temp goes high, the fan comes on at high speed.
To me it sounds like your fan clutch is a little week and not engaging properly when the temperature goes up.