Did a search couldn't really find anyone with the same issue. Heres the deal, I recently installed a aux fuel tank on my f250 PSD. Theres a check valve that lets the factory tank fill up but it dosen't let it over fill. My fuel gauge stays on full as long as there is fuel in the aux tank. My truck is now throwing a code for my fuel sensor not working properly. i dont like seeing the check egine light on all the time. is there anything i can do?

 

Here is what I theorize is happening. Your truck monitors fuel level readings, and uses that information in several ways. One is to set off the low fuel indicator. Another is a part of the calculation for miles to empty. In another way, it averages the fuel level sensor readings so that you gauge doesn't show constantly changing levels when you go up or down steep inclines.

It would appear that the truck also looks at the miles traveled and compares it to the fuel sender readings, and if there is a discrepancy it assumes there is a problem with the fuel sender and warns you, so you don't run out of fuel. I have also noticed a problem with the fuel gauge when I fill up without turning off the engine. This can result in wrong fuel gauge levels until a few on-off cycles have been completed. I have never seen a CEL light however. But because of all those functions programmed into the PCM, I think you are tricking it into thinking there is a problem.

I don't know of a work around. Hopefully someone else will have an idea. Did you contact the fuel tank manufacturer? Or did you scab this together yourself? All I can think of is that you need to run the primary tank down before you let the auxilliary tank back fill, and you probably will have to shut the truck off for a few minutes for the gauge to read properly. I know I would have to on my '06.