The fuel gauge lies: sender, float, or module

The fuel gauge is one of the few instruments whose readings are verified only by practice: filling up and running out. So its failure is usually discovered in the worst possible place.

How it works

Inside the tank sits a level sender — usually a float on an arm connected to a resistor. The float’s position changes the resistance, and the module converts that into a reading.

A consequence of the design: the sender is mechanical and wears. The resistor track erodes precisely at the positions where the float spends most time.

Typical symptoms and causes

The needle stuck at one value — an open circuit in the sender or the wiring.

The needle jumps — a worn resistor track. The most common failure, with a characteristic symptom: readings are unstable within one range and normal above and below it.

It always reads full — a short circuit or a jammed float.

It reads empty when full — an open circuit.

Readings drift in corners — normal for any tank: the fuel moves. The module smooths it, but at low levels the movement shows.

Inaccuracy in the upper half — a design characteristic of many complex tank shapes; they are not cylindrical, so level and volume are not linearly related.

Why the range estimate lies too

Range is computed from tank level and average consumption. If the first is inaccurate, the second does not save it.

Beyond that: average consumption is calculated over a previous period. After a change of route or style, the range estimate lags reality by tens of miles.

A practical rule: the range figure is a guide, not a guarantee. Refuel with margin; the parameter is covered in the live data article.

Why running on empty is harmful

Not superstition but mechanics.

The fuel pump is cooled and lubricated by fuel. At a low level it finds itself in air more often, overheats, and wears.

Sediment from the bottom goes into the filter — covered in the fuel filter article.

Condensate forms on the walls of empty space — and in winter that means freezing in the line.

What diagnostics shows

Fuel level as a parameter — in percent or litres. Compare it against the needle: a discrepancy points at the cluster, agreement points at the sender.

Level sender codes — open circuit, short, implausible signal.

Instantaneous and average consumption — to judge how well the range estimate matches reality.

An important detail: some cars have two senders in the tank (for complex tank shapes). Then a fault in one produces inaccuracy only within a certain range — which explains a symptom that otherwise looks mysterious.

What to do

1. Compare the needle against the diagnostic parameter.
2. Read the codes.
3. Fill to full and compare against the tank’s rated capacity.
4. Do not run on empty until repaired — keep at least a quarter.
5. Plan refuelling by mileage rather than by the needle while the sender is faulty.

In short

• A needle jumping within one range means a worn sender track.
• Range is computed from level and average consumption; both can lag.
• The pump is cooled by fuel; running on empty destroys it.
• Complex tanks sometimes have two senders, and one fails.
• Comparing needle against parameter points immediately at where to look.

Download the app — fuel level, consumption, and codes are read with an ELM327 adapter over Bluetooth SPP on Android.