About DTC P1800, I suggest check the harness and verify it is plugged in and still functioning.
Also, try checking your vac tubes to verify that they are not reversed. Also read this TSB with details about correct diagnostic:
DTC P1800 VIAS CONTROL SOLENOID VALVEThe VIAS control solenoid valve cuts the intake manifold vacuum signal
for power valve control. It responds to ON/OFF signals from the ECM.
When the solenoid is off, the vacuum signal from the intake manifold is
cut. When the ECM sends an ON signal the coil pulls the plunger downward
and feeds the vacuum signal to the power valve actuator.
DTC detecting condition:
• An excessively low or high voltage signal is sent to ECM through the valve
Possible cause:
• Harness or connectors (The solenoid valve circuit is open or shorted.)
• VIAS control solenoid valve
Will need replaced the solenoid valve (P/N - 149558J10A). The valve is located
under the engine cover. It's a Nissan only part so you have to order
from the dealer, either online or in person. It's cheaper online. Check this sample Nissan engine location (click image for zoom)...
DTC P1147 - Bank1 Sensor 2 O2 sensor maximum voltage monitoring
Component Description: The heated oxygen sensor 2, after three way catalyst, monitors the oxygen level in the exhaust gas on each bank.
Even if switching characteristics of the heated oxygen sensor 1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from the heated oxygen sensor 2.
This sensor is made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. Under normal conditions the heated oxygen sensor 2 is not used for engine control operation.
DTC detecting condition: The maximum voltage from the sensor is not reached to the specified voltage.
Possible cause:
Harness or connectors (The sensor circuit is open or shorted)
Heated oxygen sensor 2
Fuel pressure
Injectors
Majority of cases this is either a faulty O2 sensor, or short/open in wiring harness. Bank1 sensor2 is the rear O2 sensor on the passenger (right) hand side (click image for zoom):
Here is simple test with voltmeter to test O2 sensor. If it fails test,
replace sensor. If not, then you will have to start tracing the wiring
harness for a short/open in it (click for zoom):
And here additional details:
DTC P1147 O2 SensorKeep us updated.
