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The gas hand will not go below half a tank. This has caused me to think I have a half tank although tank is empty. What causes this please. Thanks
 

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Could be a stuck/bent sending unit. Could be bad resistors in the sending unit providing incorrect readings.


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At the rear seat, there are two fuel sensors, passenger side is the fuel pump and level sensor. The one in the driver side, is the fuel level sensor. They work together by adding the tank content. So, one of the level sensors is not going to zero, I am suspecting the one in the driver side.
 

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Replace the fuel sensor. Make sure which sensor is reading but actually is empty. Try to disconnect the connector on the drive side sensor and observe if the fuel gauge goes down.
 

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Have you unlocked your OBC? If you have, you can do OBC TEST 6 to find out if one of the fuel level sensors is not registering.

I don't know if you have the high or the low instrument cluster...but you can see in the images below of the high cluster in my e38...what OBC TEST 6 looks like:

{1st screen of TEST 6 shows the left fuel sensor/right fuel sensor/approx amount of fuel (in liters) left in tank}



{2nd screen of TEST 6 shows the amout of fuel in liters left in tank & PHASE _ (1, 2, or 3 in the blank space)


See info below about TEST 6 for more details as to what PHASE 1, 2, & 3 mean ... (again, I'm using the high cluster info because that's what I have...and I don't know what you have...the low cluster is similar but not quite the same as far as what you see in the display since the low cluster doesn't have the 20 digit alpha/numeric display for showing text etc):



 

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At the rear seat, there are two fuel sensors, passenger side is the fuel pump and level sensor. The one in the driver side, is the fuel level sensor. They work together by adding the tank content. So, one of the level sensors is not going to zero, I am suspecting the one in the driver side.
do you know what this part is called?
 

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Two (2) separate causes:

1. Sticky float...in this case use Techron or Liqui Moly injector cleaner a few times.

2. Bad Rheostat...in this case, you need a new level sensor.

Now, if you are short of budget and does not want to mess with it, then every time you fill it up, write down the mileage and after ___ miles, refill the gas. I used to own a 1985 Honda Accord with this problem, never fixed the gauge, just wrote down the mileage at each fill-up and calculate from there. The safe bet is 200-300 miles between fillups. But that was college time when I was very short of money lol...
 
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