I don't think this has been mentioned yet but the temperature light will
also flash if the concentration of antifreeze is grossly different from
specifications.  It will flash with 100% water or 100% antifreeze but I
don't know what the threshold alert percentages are for mixtures.

It will also flash if the two bare wire electrodes are dirty, greasy, or
corroded.  You might try sanding them with some fine grit paper and then
cleaning with solvent.