I'm sorry I should have supplied more information on my original post.
I have taken the m/h on two subsequent trips. I topped off the coolant twice and the level now seems stable. I'm sure the warning light signal comes from the ECM (engine control module).
But does the ECM, and the two temperature gauges all get there input signal from the same sending unit (mounted at the thermostat housing) which I replaced at the same time as the coolant?
Thanks for the previous notes and for any additional information.