Fuel Gauge always reads empty even when gas if full

My Fuel gauge on a 2007 Prius always shows empty, even after filling the tank. I performed the inclination reset and disconnected the 12 volt battery for 30 minutes. Neither procedure fixed the problem. Any ideas?

Long version: The car developed Hybrid Battery issues. I left the car parked just off the road where it sat at an angle for two months. When I went to start the car it was totally out of gas and needed a jump start. So I put a few gallons or so in the vehicle to get it to gas station and jump started the vehicle. When at the gas station I filled the vehicle while it was running as the 12v had been totally discharged. I was told the 12v will not charge if the Hybrid battery is having issues so I did not want to turn off the vehicle. I had the hybrid battery repaired, the 12v is now fully charged. Yet the vehicle still reads empty. I accidentally overfilled the vehicle since I did not understand the gauge was broken. Both Inclination Rest procedures and disconnecting the 12v battery does not help.

 

Hi Patrick, thanks for the reply. What do you mean remain READY? Will turning the car on and driving suffice?

 

I appreciate all of the responses. I disconnected the battery for an hour then filled the car with gas. Still the gas gauge shows empty. Any ideas?

 

Hi ExStudent, Thanks for the PDF. I've tried that procedure numerous times without result.

The fuel level is passed to the computer through a change in resistance. The higher the resistance the lower the fuel. Is there a way to read the resistance from the ODBii connector? That way I could "see" what the computer is receiving.

 

Hi Paul. Thanks for the reply. I hope the fix is that easy. I've filled the tank twice with no improvement. Both times I let it go down to 340 miles. It is a little scary letting it fall much further without any idea how much is actually in the tank. Both times I added 7.5 gallons of gas. Do you think draining more will help?

 

Hey I want to thank everyone for the advise. I went through the wiring diagram and figured out how the system worked. Basically fuel level info is sent via a change in resistance. Unfortunately, the resistance increases as the fuel drop. So an empty tank may be a broken wire (infinite resistance), or something else. My plan was to tear the car apart measuring the resistance readings from the gauge to the fuel tank to see if I can find the problem.

But the gauge magically fixed itself. I was on a long road trip (1,700 miles in total). After about 1,200 miles the gauge started working (after filling the car). During the entire trip, I was trying to go 350 or more miles between fill-ups with the theory draining the tank would help loosen the indicator. At 1,000 miles I overfilled the tank. Literally putting 11.02 gallons of gas. Frankly I think I was overcharged for the fuel as I should have had 2-3 gallons in the tank already. But either way, I filled the tank as full as it could get. Then about 200 miles later, I topped the tank off again with just over 3 gallons. When I turned the car on ... boom it started working.

So if someone in the future is reading this and has a similar problem, try filling the gas full, then full again after 2-3 gallons. Probably just a coincidence, but it is worth a try.