325iS Tach doesn't work- where to start?

[FDutch]

All thing works but the tach. Swap to other cars, the IC unit works fine.
Where to start? Help me please.

[Plastic Emperor]

Check fuse 21

[lennon]

What year car/engine?

[LagunaVert]

Check fuse 21

always check the cheapest thing to fix first. Keeps you from looking like a fool later.