on the cat nissan systems E43 is for air/fuel mixture ratio does not cross stoichiometrically for an extended amount of time during the intended closed-loop operation.
(Stoichiometrically means: Relationship of)

Malfunction Description:
After the engine warms up the oxygen sensor does not switch from lean to rich. It can stay at a low voltage, middle or even high voltage point.
It becomes stuck lean or rich instead of reversing directions (switching) when in closed loop.

Diagnosis:
1- clear the DTC
2- clear diagnostic code in active test
3- perform fast TAS or (relearn) air idle volume learning with engine warmed up at idle.

Note: the system will not complete the relearn if the problem still exists.

After completion of the relearn:
a) engine timing must read 0 to -1 deg
b) A/F Alpha LPG must be within specifications (75 to 100)
c) engine coolant temp must be within 83c to 94c range
d) battery volts must be a minimum of 12.5v to a maximum of 14.9v at idle.
e) Fuel pressure must be 25 to 31 kpa at idle with engine warm.
Next inspect and confirm the injection pulse LPG on the output monitor (laptop diagnostic program) between 5800 to 6900 m/sec at idle.

Areas of concern that have caused this code to set are as follows:
- leaking air horn between the mass air flow sensor and the injection holder, loose bolts possibly.
- Failed air temp sensor (readings may be off compared to actual air temp. , possibly an open in air temp circuit.
- leaking cup seal between the injection holder and the throttle body, cut or folded seal.
- blocked filter in the injection holder, a fuel restriction.

I know you already covered most of this but there may be something in here you did not.
Hope it helps :o)

good luck