Thanks Doc.
Where did you find these codes??
I have & searched EPC/WIS & AllData with no luck...

I cleared the codes and drove 120 mi. The 421 code returned.
Today, I will find the aux fan control module and investigate the fix/repair.

Update: Just looked, I dont have any Aux fans ! ! !
Did the 99 C280 leave the factory with Aux Fans??