This is a 2004 Pontiac Aztec.
I have transmission fluid in my coolant overflow tank.
What would cause that?
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That sounds like the transmission cooler has failed - the trans cooler has the hot oil passing through it next to water so the water absorbs the heat.
This may be a separate radiator or built into the main engine cooling radiator.
This needs to be checked, as if the water is getting into the transmission oil it can damage the transmission.