Get a scan tool on there to read the sensor output and see if it's lying or the gauge. If it's the gauge, it's probably the stepper motor, separately discussed. If it's the fuel level sender, 2005 was covered by a special GM program so you pay about 1/2 the dealer's cost for the repair, usually $130-150. If you didn't get a letter about the adjustment, call a local dealer with your VIN. There was a very long discussion on the OS about this - not sure we talked about it much here yet.
I went through this recently with my 05 TB; exact symptoms as you. I had the fuel sender replaced via that 50% coverage arrangement roadie mentioned, all fixed now. It is definitely a very common and widely known issue and I would suggest that should be your first course of action, assuming you don't have access to a quality scan tool to diagnose further.
IIRC, the bulletin covered 2005-2007 but expired at 10 years or 120k miles, whichever comes first. I had 82k, and just went by a dealership with my issue and they knew exactly what was going on and dealt with the 50% coverage et al without any real effort on my part. As roadie said, your cost should be in the neighborhood of $140 give or take.
If you're the handy type, you could drop the tank and replace the whole gas pump assembly or just the sender unit, GM has paid out 50% of the parts expense to some people after jumping through a few hoops. I don't know if you'd really save much doing the labor yourself but it is an option.
I had the same issues on my 07 TB i got a fuel sending unit and droped the tank with me and a buddy and it was a pain but i only paid 25 bucks for the part on ebay. so just depends if you are the handy type cause it is kind of tricky. But i saved about 100 dollars doing it. But i would bet that it is the sending unit.
Hmmm, Last night my wife just told me her gas gauge is sitting at below 'E' and her oil pressure gauge is beyond the max. Has been like that for a week or so.
This morning, I'll go out and try torque to see if I can read the fuel tank level and the oil pressure.
I'll report back here the results.
EDIT: No. Torque can't read the fuel tank level.
To clarify, I have the paid for version of Torque with the extended GM PID's plugin.
This is what I see.
This is her dash now. Yup, it's screwed.
Notice how she is using the trip counter to determine when to fill up?
I wonder if the HPTuners stuff can determine what the real fuel tank level is?
I will need help for that.
I don't know how to use HPTuners. I just bought it from PCM4L to resolve a specific problem I had.
My fuel gauge quit working in 2011. I was just using my trip to figure fuel consumption until a few months ago when the sending unit and fule line took a total dump and was leaking all over the place, of course it was one day after i just had it in for service. LOL Cost me a total out of pocket close to $400. A week later, I get a letter from GM stating they are now covering this repair for 100% fof the cost if your vehicle is less than 10yrs old and less than 120K miles. Luckily, I have on 05 which to date has only 114411 miles on it. I submitted the paperwork and within 10 days I had me reimbursment check in the mail. It was nice to see GM step up and cover all of this as I have read it's been an issue on all TB's, but also poor of them to wait so long, figuring most TB's by now have exceeded the mileage parameter for this service.
I have lost a few fuel senders over the years in my TB, sometimes they would just stop, other times they would bounce around. Really annoying to hear the low fuel thing go off every few miles!
Very often they would be low, I'd go shut the vehicle off at gas station and fuel up. Start vehicle to drive away, fuel gauge still low. Maybe it slowly would creep back up, or finally die and stay low.