My 95 Bonneville SE has decided to run the fan as soon as the key is on. It comes on low speed and then ramps up to high and stays that way until I shut the car off. Does it when cold with all the climate controls set to off. Currently don't show any codes or CEL. I did have a CEL for the A/C pressure switch but I cleaned the wire terminal and it has not come back since.

Is this another failing ECU?

I haven't heard of a failed A/C pressure switch actually causing the light to come on. If the computer detects a potential failure of the coolant temperature sensor, it defaults the fans to high. I would look at a live data scan to see what the computer is seeing and doing before jumping to any conclusions.