Usually the coolers don't go bad.  If they do it is internally (mixing oil
with coolant).  Also there is a service bulliten out on that O-ring that cold
weather will make that O-ring leak.  I believe it was a recall in Canada.  So
I would drop the cooler, visually inspect it for cracks and dents and
corrosion, if it looks OK then replace the O-ring and forget it.  Like I said
these coolers usually last the life of your motor.
My 2 cents.
Ken Wilford
Van-Again
John 3:16