My service fuel system warning came on today, along with fuel gauge going to zero and a low fuel warning.
Some background info:
I bought the car in September 2017, shortly after I noticed fuel smell and took it in to get the fuel pump replaced under the extended warranty. While they had it apart they noticed the fuel tank had a crack, so I had them replace that too. The last time I filed it up before putting it away that winter I got a check engine light. Can't remember exactly what it was, but I replaced the vapor canister vent valve/ solenoid, and it fixed it. Everything has been fine since. I recently decided to run some gm fuel system treatment plus through it to keep things working good. So I dumped in 18oz and filled up with top tier fuel. Ran it until the low fuel warning came on, and then put in $100 of fuel (which with today's fuel prices brought me up to just over 5/8 tank, I'm in Canada). Went for a drive, and after about 20 min I noticed the service fuel system warning, zero fuel gauge, and low fuel warning.
After doing a search, it seems like the common culprit is the fuel level senders, and the first recommendation is to run some techron through it. But I literally just did that. Is it just coincidence that it happened immediately after using the fuel system cleaner, or immediately after getting fuel? Is it somehow possible that it's from letting it get down low enough to turn on the fuel light? I can't see how, but my dad was with me when it happened, and he was giving me a hard time for letting it get so low, because he thinks 1/2 is empty, so he's convinced that's why. I don't normally let it get that low, I usually fill up around 1/8 of a tank, but I was trying to use all the fuel system cleaner.
Anyway, long story, but the important question is, what could be causing this? Where should I start? Thanks
Edit: Just plugged in my code reader. There's a code for fuel level sensor b. I assume that's passenger side?

 

What is the code number?

 

I don't remember. I just read it and erased it. Guess I should of wrote it down

 

After deleting the code I went for a 20 minute drive, and it was fine. Guess we'll see if it comes back

 

Went for a longer drive today. It came back after about 30 minutes. P2066 fuel level sensor B performance. Is that passenger side?

 

Yes. P2066 means that the signal from the right sensor isn't changing even though the left tank is nearly empty. This shouldn't happen because the transfer system is supposed to keep the left tank topped-off.

The code suggests you have either a transfer system problem (usually caused by an internal leak) or a stuck right-side fuel level sensor.

 

What could cause a transfer problem? And how do I tell if I have one? I did notice today when I shut it off that I heard what sounded like fluid draining back for a few seconds. Couldn't really tell where it was coming from though.
Also my left tank should be full, I'm just over a half tank

 

The car thinks the right tank is full, and the left tank is nearly empty. One way to find out if that's true is to keep driving until you run out of gas. If afterward you can only add 8 gallons, the transfer system wasn't working.

 

I don't really want to drive around until I run out of gas. Right now the car is around a half tank. Is it possible to remove the pan under the right tank and kind of lift it or move it or something and see if there's fuel in that tank? Actually, it wouldn't be possible for the right tank to be more than 1/4 full right now, since when I got gas the other day I only filled up to 5/8. Also, the left tank shouldn't be anywhere near empty, since I've only burned about 1/8 tank of fuel on the gauge. I'm not sure if that's useful info though.

 

You have to drop the tank(s) to replace the FLS. There's no way around it if it's shot. Perhaps the dealer that replaced the fuel module will offer you a discount on labor. Both FLS's should have been replaced when the fuel module was replaced previously as this is a known issue. Not saying the service dept owes you anything, but perhaps out of courtesy...a long shot for sure.

 

I know I have to drop the tank to change the level sensor, I just want to know that that is the problem before I do it. I'd hate to change it, get everything back together, and find out it was a transfer problem, and have to drop it again

 

You're close to a half tank now and don't want to run out of gas?
Jump the fuel pump relay and empty your fuel line into a gas can. If you fill a 5 gallon can I'd say it's not a transfer issue.

 

What I mean is, I don't want to drive until I run out of gas and have to take a gas can with me. I think only having filled it up to 5/8 is making this a little harder to diagnose. I never had much fuel in the passenger tank to begin with, so I can't possibly end up with a situation where is have a full passenger tank and an empty drive tank. I guess I could drive a bit more so it's under half and check if the passenger tank is completely empty.
I checked my paperwork from when the fuel pump was replaced and noticed they also replaced the lines inside the tank from the pump to the crossover line. So I'm thinking if there's a transfer issue it pretty much has to be something in the passenger tank. Maybe the best thing to do is change the level sensor and the lines inside the passenger tank? That would rule out everything except the pump in the passenger tank right?

 

If you don't want to risk running out of gas on the road, do what was suggested above and use the fuel pump to fill a gas can. The pump will empty the tank in minutes. Pour the gas back when you're done. All these preliminary tests are much easier than the actual repair.

 

I thought I had concluded that I was having a transfer issue, but then thought about it a bit more, and decided it wasn't that conclusive.
I drove a bit more, until the gauge was reading probably about 7/16, and then took the pan off the bottom of the right tank. There is still fuel. Hard to say how much for sure, but I could hear it sloshing when I shook it, and it's a lot heavier than an empty plastic tank should be. According to the math, if none transfered over since I refueled I would have about 13lbs of fuel in the tank, and I'd say that feels about right. So I thought, ok, definitely a transfer issue. But then I realized, how much can I trust my fuel gauge right now? So I'm going to drive some more, probably down to reading about 1/4 tank and then check again. But gauge seems to be dropping at about the correct rate.
So I'm definitely leaning toward transfer issue. If that's the case, where should I go from there?

 

I've been pretty busy the last couple days and haven't had a chance to play with this, but I had a chance today. So here's some more info.
I went for a drive (approx 1 hour) and all was well...until I was about 2 minutes from home, then the message came back. I checked the code, and this time it was P0461 which is sensor A. So that threw me for a bit of a loop. Anyway, the biggest reason for the drive was to burn more fuel so I could check how much was now in the passenger tank. So I took the pan off, and the passenger tank was completely empty. So I think it's safe to say it's not a transfer issue.
So I cleared the code and went for another drive, this time about 50 min. And all was well. My understanding is that if there's a problem it should throw a code after 30 min.
So I'm thinking at this point the best thing to do is go for another drive and see what happens. At that point I'll be about due for fuel, so I'll fill up and see if it's still acting up.
Another question: If I drop the passenger tank, am I kind of halfway to dropping the driver tank?

 

P0461 + P2066 + an apparently functional transfer system suggests that both level senders getting hung up. If your car was an '05 or '06 instead of an '09 it's conceivable that a dirty sensor could be spoofing one or both of these codes, but by '09 the resistance range was reversed which would require excessively low resistance for that to happen.

It would be instructive to know the actual fuel level readings as you burn through a tank. You may be able to read them with your scan tool (depending on the tool), or you could read them directly (albeit inconveniently) with a multimeter.

That said, the fix will likely require dropping both fuel tanks regardless.

If it was me, I would drop both fuel tanks and replace all internal transfer plumbing and both level senders. Your pumps are probably fine.

Before you disassemble each tank, measure the resistance of its level sender as you turn the tank upside down. See if it gets stuck somewhere. Then replace the sender anyway because it's at least 5 years old, and possibly 13 years old, depending on what was actually done back in 2017.

Replace all the transfer hoses and take care to lubricate all the O-rings in engine oil.

Before installing the tank, do the upside-down test again to make sure the new sensor isn't getting stuck. People skip this only to have to do the whole job over.

 

Looking at the receipt it looks like they replaced the left pump and the internal lines to the left pump, but not the sender, and nothing in the right. So I'm thinking I need both sending units, and the internal lines for the right tank. Does that sound right to you?
My scan tool is a cheap one that won't tell me fuel tank voltage. Can you read it with a multimeter without dropping the tanks?

 

It's been a while, but I have an update. I borrowed hptuners from a buddy so I could check the fuel tank voltages, but before I got a chance to check, it started working right again, and continued to work right for a few tanks. But then yesterday I filled up and the gauge only went to 3/4. So I checked the voltages and tank 1 was 1.53v and tank 2 was 0.71v. Guess I found the problem. I'll try putting in a bottle of techron next tank. If that doesn't work, I'll change the sending unit in the left tank this winter. Thanks for the help

 

1.53 V is too high for a full tank. The sender is either dirty, worn, or stuck.