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Your owner's manual says you have to fill the rad to the top and fill
the reservoir to MAX anytime there's work done on the cooling system. If
you don't, the air can't be displaced and your cooling fan may not cut
in and you can warp your head.

Honda premix is best, and check it next day after it cools. Top up again
if necessary.

'Curly'

Bad thermostat.

If you bought one aftermarket, it may very well be scrap right out of the
box.  I have had many over the years that wouldn't open, or wouldn't close
when brand spanking new.

Also check your coolant level.  If it is not full, and to the max level on
the overflow bottle, this could be your problem.