Service Engine Soon then fuel gauge empty to full

[bighank]

Got the equivalent of a check engine light "Service Engine Soon", and went to have it scanned with an OBDII reader. While driving the gas gauge started going from full to empty and near empty the gas tank light showing low fuel illuminated. Stopped at gas station and looked under the car to see if any gas was leaking. No leaks. Filled up tank, took only 2+ gallons, and gas gauge still at empty. Drove about 3 miles and gauge started moving from empty to full.
Stopped at AutoZone, Advance Auto and Pep Boys but all 3 stores had "lost" their OBDII readers. After last stop was driving back home and Service Engine Soon light went out.
The car seems hexed.
Now don't have any codes to read and gas gauge right now with a super full tank reads full.
Any ideas on this one?

 

[sergeimoskva]

Hi !
The sending unit works by varying the resistance via a potentiometer attached to a float in the take. As the float rises, so does the resistance. Empty it should be around 40 ohms. Full it should be 250 ohms.


First is check the resistance across the two wires at the connector under the second-row passenger seat under the car(driver's side - see above image). It should be between 40-250 depending on the level of fuel. If it is, then it's not the sending unit. If it's not, then you'll have to drop the tank to get to the connector there (or to change the sending unit anyways). It's not hard, just awkward especially if there is a significant amount of fuel (6 lbs/gallon).



If there is no or very little resistance, you have a short, most likely at the sending unit. If you have significantly more than 250 ohms of resistance, then your sending unit may be shot. If you have infinite resistance, then you have a broken wire or a bad sending unit.
If everything test fine at that connector, go to the connector at the firewall beneath the dash. Measure the resistance there. If you measure the same as you did with the connector under the car, then your wiring is fine there. Otherwise, your problem lies between the two connectors.