take out the sensor and make sure the metal points are clean.
I had a cracked expansion tank along the threads of the cap of the tank. You could not really see it - but it was enough to leak the coolant and trip the sensor.

Pray it is not the beginning of a leaking head.

Todd

 -------------- Original message ----------------------
From: John Runberg <jrunberg@MAC.COM>
> We'll, now that the fuel system on my '90 is sorted, the coolant system wants
> attention.
>
> Yesterday the coolant light started blinking. Needle mid-way, rad fan works but
> wasn't even on (checked function afterwards. Works correctly). Engine at
> operating temp, but not overheated. Thin bit of coolant on the front lip of the
> expansion tank (left side of engine, not the overflow). Level in expansion tank
> looked a little low, just at the min line. Expansion tank looked a little high,
> just over the max line. No apparent loss of coolant previously.
>
> I know of at least two things that are bad / suspect, but would like opinions on
> other possibilities.
>
> 1) expansion tank cracked. Today, after a short drive, could see coolant
> emerging from just above the lip on the expansion tank, then dripping onto the
> header.
> 2) thin (9mm?) line coming from expansion tank to overflow tank looks suspect,
> possibly collapsed inside.
>
> Additional questions:
> - does it make sense to replace the expansion tank cap, too?
> - same deal w/ the level sensor - replace?
>
> Both these items are getting replaced, but would like to know opinions before I
> continue.
>
> Much thanks :)
>
> john