In have a 2001 fuel injected 454 Mag and the IAT is reading -40 at the computer with a scanner. There are no engine codes and I have already replaced the sensor. Testing the wires at the sensor the tan wire has 5v and the black wire is grounded. The wiring between the sensor and the computer appears to be good. I am not sure what to check next.

If it is like the ones I see on cars I am not sure if the black wire really should be 100 % ground, as in Zero Ohms. Are you checking it on the lowest scale on your meter ? I would unplug the harness from the computer and check the black wire to ground again.

You can also hook your ohmmeter leads across the sensor plug and heat it up with a hair dryer and see if the resistance changes. Just in case you got a defective one.

Likely an open circuit. My car did the same thing when wire broke inside the insulation. I would check continuity between sensor plug and computer. Be sure to flex wiring while testing in case it's intermittent, especially on sensor end of wire.

Intake air temp sensors are typically just 2 wire senders right ?

-40 means open circuit/ no connection/ broken wire/unplugged. Unless the IAT is physically damaged they rarely go bad on this style. Since you have a scanner, put a jumper wire or paper clip acrossed the two female pins with ignition on and it should swing from -40 to other end of IAT table which I think is 200 degrees or 96 C or something. Your sure your on the IAT right? (towards rear in upper plenum)