My Pathfinder ST (Australia version), since new the needle of the tachometer always fluctuate up and down small amount.

Not normal, at least not for me, my 2014's tachometer stays pretty much still whenever idling.

Also, when turn on rear air by pressing the REAR at front instrument, the fan speed will automatically goes up to max, any one experience this too? It is abit annoying to have to turn the fan speed down.

Have you checked the temperature setting for the rear? It doesn't happen to me so I can only imagine that the rear temperature is set far lower than the front so the fan maxes up the minute you turn the rear on.

 

mine rear a/c always turn to auto temp as soon as i turn it on, so i always need adjust the temp i want so it will cancel auto temp.

 

So your idle doesn't flutuate, I need to go to the dealer and test drive other pathfinder. The bloody tech guy said it is normal. My rear temp is always at the lowest same at the front

 

Do this sound like it could be your problem? Have your dealer take a look at Nissan Service Bulletin NTB15004...

Summary of NTB15004:
IF YOU CONFIRM: One or more of the following with the engine idling, the CVT in Drive, and the air conditioning on: - Low/rough idle - High idle - Idle fluctuation ACTION: 1. Compare the vehicle’s current ECM part number to those listed inTable A (see page 4). 2. If the ECM part number matches one listed in Table A, reprogram the ECM. - If there is NOT an ECM part number match, this bulletin does not apply. Refer to ASIST and the Electronic Service Manual (ESM) for further diagnosis.See this bulletin for further detail.