Faulty Fuel Gauge

A common question and problem for vehicles as they age is for the fuel gauge to begin registering incorrectly. Sometimes it is "a few gallons off" but usually it will register full and then suddenly empty after a few miles of driving after a fill up. Nearly every time, this is because the sending unit (aka "fuel gauge") in the fuel tank has become faulty.

Of course, assuming something and then throwing parts at it is the surest way to waste time and money. So doing proper diagnostics before buying parts for replacement is important.

Recently, in our Ask a Mechanic forum, someone had a problem with the fuel gauge on their 2003 Pontiac Aztec. A few minutes of diagnosis and a couple of minutes doing a basic Web search easily solved the problem.

To start with, do some basic research on the vehicle itself via the Internet. Google the year, make and model (in this case "2003 Pontiac Aztec") along with "fuel gauge" (i.e. "2003 Pontiac Aztec fuel gauge") and see what comes up. Many of the results will be shopping hits, of course, with sources to buy gauges, but others will be to forums and repair sites. A quick read of those informs us that the Aztec has a regular history of bad fuel sending units (in-tank gauges).

So we can now be more sure it might be the sending unit.

Before replacing, however, some basic and easy tests should be done. Sometimes, the sending unit is OK, but the connections to it are not and it's "wiggling" or vibrations during driving that are causing the dashboard gauge to misread the tank level. Most of the time, this will mean either the dash gauge doesn't change at all or fluctuates during driving (going up and down). Often, on newer vehicles, bad connections or connectors will result in engine codes being sent to the computer, so checking those is a good idea.

Complete circuit tests of the connections from the sending unit (mounted on top of the gas tank) to the dashboard gauge (or computer) should be done. Often, the problem is a bad connector, especially the one mounted directly on top of the sending unit, outside of the fuel tank. If that is the problem, it's much easier and cheaper to replace than the sending unit itself (on models where they are separate pieces).

Once everything else is ruled out, you can change the sending unit. This is a relatively time-consuming job that requires some skill, but it is doable by most competent home mechanics. Most of the work involves gaining access to the fuel tank. On some vehicles, this is as simple as removing the bottom portion of the rear seat. On others, the tank will need to be "dropped" (mostly removed) from the vehicle.

Once access is gained, removal of the unit is straight-forward. Replacing should also require putting in new O-rings and seals, which are usually part of the new sending unit kit (make sure before you proceed).

Other precautions should also be made, since you'll be exposing (opening) the fuel tank, which will be full of flammable and noxious fumes. Proper ventilation and gear are a must.