[Richard L]

This may not work because most ECUs have an internal pull up resistor tied to the IAT sensor. If the circuit goes open circuited, the pin will to up towards 5V instead of OV. This will make the ECU think the IAT is very cold.

You need to ground the black wire of the grey harness upon failsafe activation. Make sure you don't link the DR1 or the dummy resistor will get very hot and eventually melt the controller box. White wire t-splice into the tan wire, no need to cut the wire.

Instead of ground the black wire, why not place a 47-100 ohm resistor inline so you can actually simulate a known temperature (hot). Some ECU may throw a CEL if the reading is out of range.