2007 Jetta 2.0 TDI Radiator Fan problem

[PaulBn]

The radiator has 2 fans mounted in front.
The larger one on the left is working especially when the AC is on.
The smaller one does not spin at all.

What is the best way for me to investigate further?

Thanks, Paul

[niall campbell]

put a live feed to the fan to ensure it spins

check that the wiring has live going to it with engine running

check water temp sensor as well

[niall campbell]

that's the water temp sensor in the radiator