Given that I had the Bentley manual and the TSB, it was a very easy job; maybe a 2 on a scale of 1-10. The manual and the TSB told me everything that I needed to disconnect and how to disconnect those things. The toughest bit is unscrewing the ring that holds the sending unit in the fuel tank. It takes a special tool, so you'll either have to get the tool or get creative.

I have this problem too but in my case the gas tank is not actually full and the pump is just auto-shutoff. I found this info about it:
http://www.986faq.com/10-0/problems.asp#018

Mine is a 987 2007, the dealer says they can't reproduce the problem so they wont fix it. anyone have this issue with a 987?

Simple And Easy Fix! Works Every Time.
 

I found the easiest and simplest way to get the correct reading NO MATTER WHAT MODEL BOXSTER you have is to fill your tank up with the key turned to the far right, the "ON" position that will display your readings. Of course do not turn the car on so the engine starts. Simply fill your tank up and you can watch the gas gauge fill while you sit in your car. You will never get a false reading this way and it will "FILL UP" to the full mark on the tank reading this way. :cheers:

You're mistakenly assuming that the fuel sending unit is functioning correctly, which it doesn't seem to be in certain cases, hence the TSB discussed in this thread. Incidentally, mine must have somehow freed itself (temporarily, at least) and finally registered a full tank at my last few fill-ups. I never thought I'd be so pleased to simply have a fuel gauge that works. :D

Just to add what I'm learning about this issue, which my car has as well (full = 5/8 reading), I read that there is a fuel tank vent valve can be bad or the fuse for it (E8??) has burned out.

I have also discovered that my tank wasn't totally full when the pump clicked off automatically. I had to dribble in the last gallon or two to get it to fill up and raise the needle to 7/8.

However, I still have a problem with an accurate reading. Just once since I replaced my sending unit and had it calibrated by my mechanic (requires a PST tool) has the car read 4/4 when it was full. Grrr!

One other issue is that when I am at 1/4, the low fuel light comes on brightly. Seems a little premature, even though one should not run to empty in our cars (remaining fuel keeps the fuel pump cool and from overheating).

I have yet to check my fuses, but I just might go home tonight and make a bee-line for the garage to check this out.

This problem has always frustrated me about owning my car!

On my Boxster the auto shut-off occurs at approx. 7/8 full. On level ground if I run about two more squeezes into the tank it will actually go past the "full" line. I try to keep the tank over 1/4 full since I don't trust the fuel gage. Recently I did run it down till the near empty light began to glow. When I filled it the car took 13.4 gallons.

Fuel Gauge Reading
 

got delivery of the 98 with 1/2 tank. Driving and gauge steadily going down. When the red light came on, I filled the tank with gas. The gauge never indicated a full tank but read about 1/8 and after 30 miles, red light came back on. I know the gas went into the tank.
With the Durametric, I can get the gauge to read 1/2 and full, so I guess its the sender? Why did it work until I filled the tank. I did swervy thing like warming the tires to see if I could shake it loose, but no.