2001 Nissan Pathfinder 3.5l 4x4 P1320 code accompanied by all four O2 Heater circuit codes

The codes for the O2 sensors are for the heater side which means you should be testing the power supply to the O2 sensors. It could be a fuse problem.

P1320 is more likely to be related to the ignition coils and test the resistance for the coils first.

Thanks for the quick reply. I looked at all of the fuses that I thought my be related to the O2 sensors, all good. Are there any fuses other than the main ones under the hood? Is there a common connector that I can check power to those sensors? With it running in open loop, could that cause the poor gas mileage? What is nominal for highway driving on this model/year?

For HO2S testing.

Procedure

1. Turn ignition off. Disconnect appropriate front HO2S harness connector. See
Fig. 12 or Fig. 13 . Turn ignition on. Measure voltage between ground and
appropriate front HO2S harness connector terminal No. 1 (Red/White wire). If
battery voltage exists, go to next step. If battery voltage does not exist, repair
open or short circuit and retest system.

2. Turn ignition off. Disconnect ECM harness connector. See Fig. 3. Check continuity of
Blue/Orange wire between right front HO2S harness connector terminal No. 3 and ECM harness connector terminal No. 3, or Red/Green wire between left front
HO2S harness connector terminal No. 3 and ECM harness connector terminal No.
2. See Fig. 4. Also check circuit for short to ground and short to voltage. Repair circuit as necessary and retest system. If circuit is okay, go to next step.

3. Turn ignition off. Leave appropriate HO2S harness connector disconnected.
Measure resistance between HO2S outer terminals. Resistance should be 2.3-4.3 ohms at 77°F (25°C). Check continuity between both outer HO2S terminals and
center terminal. Continuity should not exist. If HO2S does not test as specified,
replace HO2S and retest system. If HO2S is okay, go to next step.

4. No problem is indicated at this time. Problem may be intermittent.