4 Reasons Why Your Gas Gauge Isn't Working

How the Gas Gauge Works

The gas gauge system can be broken down into three basic parts: gauge, sender, and circuit. A fault in one or more of these parts will lead to gas gauge failure.

The sending unit is usually part of the Fuel Pump Module: a combination of parts that includes the fuel pump, fuel strainer, fuel filter, and fuel float. Input voltage, typically from the ignition circuit, is modified by the fuel sender. The fuel float contacts run on a potentiometer, or variable resistor, whose resistance changes depending on float level, effecting a change in output voltage. 

Some systems are wired so that high fuel level contacts the low-resistance section, gradually increasing resistance as fuel level drops. The system thus outputs higher voltage at high fuel level, gradually dropping voltage as fuel level drops. Other systems have opposite wiring (high fuel level corresponds to high resistance and low voltage) but nonetheless go through the same process. 

The gas gauge circuit connects the battery, sending unit, gas gauge, and ground. Most modern sending units are grounded to the electrical system, but some older cars were grounded to the body or frame.

The gas gauge in the instrument cluster is the visual indication of activity in the fuel tank and sending unit. Some gas gauges are directly controlled by voltage feedback from the sending unit, while others are controlled by the instrument cluster, which itself gets voltage information from the sending unit.

Common Gas Gauge Issues

The gas gauge is a relatively simple circuit, but its simplicity means each component is essential to its function. Here are four ways that the gas gauge can fail.

1. Sending Unit Failure is the most-common cause of a gas gauge not working. When the vehicle is in motion, the sending unit is in constant motion, constantly rubbing the variable resistor. Over time, the contacts can wear, leading to an open circuit. The gas gauge might interpret voltage feedback from a dead sender as FULL or EMPTY, consequently pegging the gauge no matter the actual fuel level.
2. Circuit Problems can cause the gas gauge to stop functioning normally. Depending on the location of the fault, the fuel sender may not have a source voltage, the gas gauge may have no fuel sender voltage, or the ground for either one may be interrupted. Loose connections and corrosion can also cause problems, particularly at the Fuel Pump Module, which is usually exposed to the elements.
3. Gas Gauge Failure is less common, but still a possible issue. If the internal circuit is faulty, the gas gauge may only function in one section, such as between HALF and FULL or between EMPTY and HALF. If the internal circuits are shorted, they may peg to FULL or EMPTY. If the circuit is open, the gas gauge will likely sit at EMPTY and never move.
4. Instrument Cluster Failure is the least common, and likely the most expensive problem to fix. Modern instrument clusters are fully-integrated circuits and may not even feature replaceable bulbs. If the gas gauge fails as part of the cluster, the whole unit must be replaced.

How to Identify the Source of a Gas Gauge Problem

Before you begin testing your gas gauge, gather the following tools: an electrical wiring diagram (EWD), a digital multimeter (DMM), and basic hand tools. Then, run through the following tests to determine the source fo the problem. 

1. Instrument cluster self-test. Many modern cars and trucks are equipped with an instrument cluster self-test feature to test computer-controlled instrument clusters. The procedure may be in your owner’s manual or available online (enter “[make] [model] instrument cluster self-test” in your favorite search engine). The test puts the instrument cluster through its paces, testing the digital lights and readouts as well as swiping the gauges through their ranges. Pay special attention to whether or not the gas gauge sweeps smoothly from EMPTY to FULL. Note that some self-test steps may stop the gas gauge at 1/4, 1/2, and 3/4, as well.
2. Fuel sender test. The fuel sender test should be conducted when the tank is lower than HALF, to prevent fuel splashing. Start by making sure the plug is clean, dry, and free of corrosion. Make sure the pins are straight and that the connector is fully seated. Remove the fuel pump module so you can manipulate the float arm. At this point, with the key in the ON position (but without starting the engine), back-probe the connector and check for voltage. You should always have 5 V or 12 V on one of the pins. One of the other pins will be voltage feedback to the gas gauge. As you swing the float arm, output voltage should increase or decrease smoothly, depending on whether you’re moving the float up or down.
   If the input voltage is incorrect, check the circuit between the sending unit and the ignition or its voltage source. If the output voltage in incorrect, then you likely have a sending unit problem. If input and output voltage are correct, then you likely have a circuit problem between the sending unit and the gas gauge.
3. Gas gauge test. When testing at the gas gauge, repeat the voltage check from the sending unit. The voltage should be exactly the same as when you tested at the sending unit. If the voltage is different, you likely have corrosion or poor wiring between the sending unit and the gas gauge.

These steps should reveal the problem, but be wary of modern computer-controlled instrument clusters and gas gauges, which can be confusing to diagnose and repair. It’s always a good idea to check with a professional when diagnosing complicated systems in order to avoid expensive mistakes.