Heater core has failed.
The DeVille could have low transmission. Or the internal transmission bands or valves could have failed. The transmission will most likely need a rebuild.
Well... mines doing the same thing. 88 sedan deville, 4.5 ltr v8. It turns over but I watch and nothing is coming through the injectors... no fuel. It happens off and on, sometimes it works and sometimes it doesn't. I think it might be the fuel filter but i can't find that thing.
O2 sensor, and catalytic converter can be ruined. You can also cause internal damage to the engine, depending on what the problem is. Have it repaired.
Since they run in parallel, there's no way that a heater core will cause a radiator to get hot.
It relates to and O2 sensor heater circuit failure. The ECU measures resistance through the heater wire, and if it breaks it will produce that code. Dirty or corroded contacts can also cause the fault.
I daily drove mine in Iowa in the summer and winter from 2005 to 2011 and got a fairly consistent 9 mpg.
If the engine is getting up to normal temp, the heater core might be plugged. Heater core is not plugged and normal operating temp!
A leaking heater core will cause the engine to overheat because the coolant is leaking out.
There are several things that can cause your Cadillac Deville to have transmission problems. The most common transmission problems are worn transmission gears.