On my 1987 vanagon with stock 2.1 engine, the red LED embedded in the Coolant Temp guage started flashing a few kilometres from home this morning.  Stopped and had a look, the expansion tank was completely full and my new pressure cap had properly pulled down the level in what the Bentley calls the Refill tank.  The Temp guage itself was reading normal. So I drove home with the light flashing all the way.

At home, I waited until it cooled and removed the pressure cap.  Sure enough, completely full.  So what is causing the flashing red light?  Is there a way to test the sensor and the wiring? The Bentley is not much help here.

All suggestions welcome.
Rick