from the failures and leaks ive seen 50% (ISH) fail on the drive, you either spot a puddle and go looking, or the low coolant alarm sounds then you check etc. or you do your routine checks.
the others.
if you have a slight leak in a LOW pipe/waterpump etc, air can be drawn in when the motor cools, this can go unnoticed for weeks as a small amount of coolant is lost when the coolant is warm (pressure up) and it sucks in air when cooling down, this repeats until the front matrix and maybe the head is full of air, at this point coolant flow is lost and it super heats in the head.
the low coolant alarm is not triggered as there is no fault to pick up as the coolant is held up to level with the replacing air.
these slow leaks are the hardest to find and fix.
the high coolant alarm can warn of the expansion of the coolant that is outside its normal range of expansion just as it starts to boil and blow the saftey valve (the cap). or if more air is admited than coolant lost the level can rise. this may buy you vital time to pull the motor over and stop it when and if the temp gauge moves from its sleepy 11 oclock position.
if you have a low coolant alarm already it may seem right to add on the high alarm.
while on the subject of coolant boiling and temps the blue coolant usually has a lower boiling point than the red, for this reason i only fit red coolant-----a little extra protection/time.combine this with a good coolant temp monitoring gauge and you are as well protected as is practical.
not that we are all paranoid or anything
just my opinion---for what its worth.
. To anyone who is unsure it they need one, I would definitely recommend it. My home-made high coolant alarm was able to detect a tiny amount of air that got sucked into the system due to a small nick in the end of the bleed pipe. After cutting off the damaged section & re-bleeding, the van has since covered overed 10,000 trouble free miles.