I think Willie's correct. fwiw: earlier this year I apparently lost a few cups of coolant when doing a repair,, the temp gauge in the car indicated nothing, but the OBDIICC gauge showed my temp spiking radically [steam] then dropping to a bit higher than normal, [95C vs. normal 91C-92C] and the MIL
appeared right at the same time.
is on youtube.
Highwater's posted on IC a great diagram of the first gen's coolant system and it is mirrored here
if you want to check for leaks.
If you think it's an internal leak, ie. blown head gasket, you can note this in the purge watching for bubbles arising that may vary with rpm; or after the car's warmed up, holding a kleenex over the exhaust and watching the tissue disintegrate with all the excess water vapor. Also uf far gone, white smoke in exhaust. I hope it isn't that bad.
fwiw: Never check rad fluid or top up via the rad cap as it's lower than the highest point of the coolant system.