VY overheating - fan problem?

[jimmyVY]

Hi guys
VY Calais S2 V6 overheating when stopped at lights with AC on - temp quickly returns to normal with good air flow.
Coolant levels were good, flushed the radiator, no oil in water or water in oil.
Then I noticed only one of the dual radiator fans (left side) ever seems to be going once the engine warms up.
How do I diagnose problems with the fan? Broken electric motor or dodgy wiring?
Other thoughts are to replace the thermostat or give block a complete flush.

Any thoughts guys??

[Burnout124]

only one fan will work at normal temp the other starts when the car has sarted to boil. thermostat is a good place to start along with a complete flush