First, Dex did not eat your radiator or heater core. Dex is good for 5 years, or 100k miles, which ever comes first.
If the radiator was topped off with a coolant that is not compatible with dex, then there will be problems. It will sludge up, you will find a black sludge. Do you see this? What color is the coolant? Dex is a pink or orange color.
If it'* still under warranty, then they need to cover it. They will however try to find any excuse to get out doing the work. You may want to read over your paperwork.
SAVING THE WORLD ONE BONNEVILLE AT A TIME
1997,1998,2000,2005 year confused Bonneville SSExi. Stock? ish.
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