If that fault code is as you say "circuit malfunction" it should be a purely electrical fault.
So, has the car had a prang recently, or any other work that would see the wiring messed with? It's possibly a dodgy earth somewhere. Or even as silly as recently jump started. Or a leaky windscreen letting the ECU get wet.....
What I'd be doing is grabbing a workshop manual, and work through the diagnosis: Which will be:
Read the sensor reading in the ECU and checking the wiring. I'd be finding where the sensor get's it's power from, confirming that's good, then confirming the earths good, then strap a multimeter across those two points and play with the wiring.
[EDIT] Ah, so you've tested the circuit to the ECU, did you check for short to earth and power?
Intake air temp sensor is a stand alone circuit (in most case, I would think a Jap car it would be certainly).... Although! Some AFM's have and IAT in them also, it's not a AFM fault is it?
Where as if it's just a air temp parameter fault (ie. not circuit malfunction), it could be a coolant temp sensor isn't right, but the ECU interprets that the IAT is wrong etc..
I hate modern faults, their codes and misleading names: Determing what the fault is, and what the symptom is can often be a bigger battle than the repair.
For example: I recently got caught on an audi, the A/c wouldn't work, the fault code was system pressure too high, but the fans weren't working... So were the fans not cooling the condensor causing the high pressure (which it will) or was the pressure sensor wrong (very very common) and not triggering the fans.... Both the fan control and the sensor out put are duty cycle, so you can't really test either signal without the correct expensive equipment, even then you'd need to know what those signals for that model need to look like....... Lot's of thinking... measuring temperatures with laser thermometers etc... and enventually I was nearly 100% sure it was the fans... but then, was it the fans themselves, or the speed control module, or the wirings.... yada, yada, yada.... eventually I was almost convinced it was the fan speed control module (which was hard wired in the loom, yes very expensive).... swapped it out... Bingo, problem solved, phew.... There was always a chance it was the pressure sensor, that's just how the fault detection systems work, their bloody dumb.