Frosty,
Had no warning at all. No smell, fog or anything. In fact when it lt loose, my first thought was a fire and smoke, not steam. Luckily it was steam.
$129 is a fair price for a core but $700 is also fair for a core plus installation. I had a Ford Taurus as a company car years ago and it cost $1200 to replace that heater core.
Anziani