P1400 no visible problem some gas milage issues

Hi Richard:
If there is a lot of pitting or corrosion visible, it would not hurt.
Replacing the spark plugs and air filter would help.
Changing engine oil and oil filter may not help with milage but it will make the engine last longer.
Increasing tire air pressure to the maximum rated for the tires will usually result in better milage but may actually increase tire wear rate.
Cheers

Problems with the intake manifold, a vacuum leak in the exhaust and proper gas recirculation can produce these codes. p1400 codes,

Most likely a failed exhaust converter, that should have lasted
12 to 14 years

Google your codes

So that motor has had issues for years, I would guess

Cylinder Misfires,Rich Fuel Trim ,neglect or something

Have to fix all it's problems, then replace the converters

P1400 NISSAN - EGRC Solenoid Valve
Possible causes- The EGRC-solenoid valve harness is open or shorted
- The EGRC-solenoid valve electrical circuit poor connection
- Faulty The EGRC-solenoid valve

These trucks with the flex fuel systems will not get as good of milage on E85. They are designed this way. I haven't heard of any kind of spark plug that will help you in this. It is kind of a trade off, better milage but gas is more expensive, E85 is cheaper and more enviromently freindly but gas milage suffers. I also have a 2006 Chevy but did not get the flex fuel for this reason. On my car I use it and the milage suffers a little but with the trucks you will see even less milage due to the fact that trucks get worse milage than cars to begin with.

P1400 Exhaust Gas Recirculation valve position sensor
P1401 Exhaust Gas Recirculation valve position sensor
P1402 Exhaust Gas Recirculation metering orifice restricted
P1403 Exhaust Gas Recirculation differential pressure sensor
P1404 EGR Temperature Sensor circuit
P1405 Exhaust Gas Recirculation Differential Pressure Feedback Sensor upstream hose off or plugged
P1406 Exhaust Gas Recirculation Differential Pressure Feedback Sensor downstream hose off or plugged
P1407 EGR solenoid -No Flow Detected
P1408 EGR solenoid -Flow out of Self- Test Range
P1409 EGR Vacuum Regulator solenoid circuit malfunction
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The sensor for the EGR Valve position is not functioning correctly.

Exhaust gas recirculation flow insufficient detected.
Maybe you got a bad DPFE sensor, or the pintel inside the egr valve is not opening all the way causing the insufficient flow.

Gas milage will vary, depending on the manner in which you drive, also poor gas milage could be due to poor maintenance. Replace the PCV valve, the oxygen sensors, the fuel filter, plugs and wires, and have the injectors cleaned, switch to synthetic oil should improve the milage. .

Not sure where you live but if the gas you get has a higher amount of ethanol in it you will get worse gas milage.But there are many reasons that effect gas millage.