Handy knowledge! I figured my girlfriend was gonna kill me if I kept killing the engine at lights, heat cuts out DAMN fast in the Saturn with the engine off. I think it's because the heater controls are vacuum operated, no engine, no heat.

Aha - I think I saw this on cleanmpg.com

I believe you need to set the ScanGauge to "Hybrid". This allows it to track distance and register zero (instantaneous) fuel consumption. Yes, that's for non-hybrid cars. Hybrids of course, you also set it to "Hybrid".

https://www.cleanmpg.com/forums/revie...e-ii-4495.html

Quoting from there:
The next setup parameter from the detailed setup screen that needs to be adjusted is “Fuel”. Push the button next to “MORE” ---> Push the Button next to “SETUP” ---> Push the Button next to “FUEL”.

1. Press the button next to “TYPE”.
2. Select the type of vehicle by pressing the top buttons on the right or left to show Gas, Diesel a, Diesel b or Hybrid. If you do not own a diesel, select Hybrid even if you do not own a hybrid for the ability to continue recording data points through a FAS. [Note: "FAS" = EOC]
3. Press the button next to “SAVE”.
4. Press the “Home” button which will bring you back to the initial Selection screen.

Like so....

The heat works because the water pump sends HOT antifreeze / coolant through the heater core, which is basically a small radiator, so it has fins which heat up. Then your interior fan blows air through that heater core, the air absorbs the heat from the fins and this creates warm air.

When you turn the engine off, the water pump stops and your coolant stops circulating.
It takes only seconds for a fan to cool off what hot water is left in the heater core, doesn't matter what vents it's blowing out of, without HOT coolant circulation you have no heat.

Try instead turn your interior fan OFF when the engine is off.
Or keep the key out of accessory mode.
Shouldn't get nearly as cold now.