Fuel Level Sensor

Most sensors use a float on an arm that moves a sliding contact over a resistor track. The resistance changes with the fuel level and the instrument cluster or a control unit converts it to a reading. The engine control unit also uses the fuel level: OBD-II only runs some EVAP leak tests within a certain fuel level range, because a nearly full or empty tank distorts the result. Most sensors use a float on an arm connected to a variable resistor. Some tanks with complex shapes have two sensors, one on each side.

The float rises and falls with the fuel. Saddle-shaped tanks on all-wheel-drive cars often have two senders, one on each side. The instrument cluster or control unit damps the signal so that the gauge does not jump around with fuel sloshing in corners or on hills.

A worn sensor causes an erratic gauge, a gauge stuck at full or empty, or codes like P0460. Never rely on a faulty gauge: running out of fuel can damage the pump and, on diesels, need bleeding. A faulty sensor can make the gauge drop suddenly, stay full or read empty. Until it is fixed, keep track of distance since refuelling so you do not run out unexpectedly.