So the odometer screen is showing only 8's and is not working right. I can't seem to find anyone that has faced this problem, and the garages here are telling me to replace the whole gauge.
Problem is, the mileage is usually stored on the gauges, and if I swap to a used one, I won't get the actual mileage.

Can someone please help?
This is how my gauges currently look like: https://drive.google.com/open?id=1F_...-vT2BY-Dt5oqQa

It's an 04 Coupe MT6.

 

I would be more concerned with having a functional instrument cluster rather than having an accurate odometer reading. Definitely looks like you have some circuitry that broke in the cluster.

As for keeping the existing odometer reading, you would need to disassemble down to the PCB, find the EEPROM chip, desolder it and solder it to the new instrument cluster.

Alternatively you can take it to Nissan / Infiniti and they can order a new instrument cluster with the correct odometer reading, new odometers are Nevermind it's a discontinued item, the service department used to be able to order a new cluster with the correct odometer reading, it was like $1500 for a new instrument cluster plus 5-6 hours of labor at $125/hr.

 

That's almost how much I paid for the car, so that's off the table.

Just need to know which chip is the EEPROM and transfer it

 

1. I would resolder any points that look cold or not shiny. Cold solders crack over time. It wouldnt hurt to resolder (reheat) any solders you can.

2. A quick search shows a few cluster repair ads on line.
that may be your best bet, should you be able to afford some down time. Others appear to give you fast turn around by letting you purchase a repaired one with correct odometer reading and they keep your original cluster (core).

see post #9
https://g35driver.com/forums/general-tech-questions/446383-time-get-instrument-cluster-repaired.html

https://g35driver.com/forums/g35-sedan-v35-2003-06/464677-instrument-cluster-reset.html