How to Accurately Diagnose a Bad Fuel Gauge

Are you thinking you have a faulty fuel gauge, and you’re not sure where to start diagnosing the issue? We’ll help you identify the underlying reason(s) your fuel gauge is acting erratically.

Fuel gauge

Not Sure if Your Fuel Gauge is Faulty? Try This.

Wondering if your fuel gauge needs attention? There’s an easy way to find out, and it's to compare your gauge to the amount of miles you’ve driven. This one requires some simple math.

The next time you fill up your car, write down your mileage. Once your fuel gauge hits empty, write down your mileage again. Next, subtract the old mileage from the new mileage. The result is the amount of miles you’ve driven since you last filled up your gas tank.

Now, write down your car’s gas tank capacity (in gallons) and its average fuel economy. Divide the number of miles you’ve driven by your average fuel economy, and you have the number of gallons of fuel you’ve used since your last fill-up. Take that number and divide it by the tank capacity, and then multiply it by 100. That’s the percentage of fuel actually used compared to the fuel that’s supposed to be used.

(New mileage) – (old mileage) = (number of miles driven since your last fill-up)

(Number of miles driven) / (average fuel economy) = (gallons of fuel used since your last fill-up)

(Gallons of fuel used) / (tank capacity) * 100 = (percentage of fuel used vs. expected use)

If you’re still confused, let’s use some real numbers:

Let's say you have a car with a 25-gallon tank, and your average fuel economy is 20 miles per gallon. You’ve driven 200 miles since your last fill-up. If you take 200 miles and divide it by 20, you’ll find that you’ve used 10 gallons of fuel since your last fill-up. That’s 40% of your total tank capacity (25 gallons). If your fuel gauge is at empty, then you know you’ve got a problem. Its reading should be at just over half, not empty.

If the percentage is close to 100%, then your fuel gauge is working correctly. If it’s under 80% then you might have a problem.

Diagnosing Your Faulty Fuel Gauge

The symptoms can tell you a lot about what’s going on with your fuel gauge. Use these simple yes-and-no questions about the symptoms to help you narrow down the possible reasons your fuel gauge is going haywire.

Ohmmeter

An ohmmeter, via John Adams

1. First , take a look at your temperature and oil pressure gauges. Are they acting erratically, too?

This includes the needle staying in the same place, flying all over the place, or giving an obviously inaccurate reading.

  • Yes: The problem lies in the instrument voltage regulator or instrument panel wiring. Move on to question #2 to determine if that’s indeed the case.
  • No: The problem lies in the fuel gauge itself, the sending unit, or the wiring in between. Jump ahead to question #3 to figure out what to do first.

2. Check the voltage output of the regulator. Is the reading about 5 volts?

Your owner’s manual should tell you how to find the instrument voltage regulator and check its voltage output.

  • Yes: The instrument voltage regulator and the wires in the panel are okay. The issue is probably the fuel gauge, its sending unit, or the wiring in between.
  • No: We know what the problem is! The instrument voltage regulator has a weak ground connection or bad resistance wiring.

3. Do you have to drop the fuel tank to reach the sending unit connector?

  • Yes: It’s easier to troubleshoot the fuel gauge wiring first. Jump ahead to question #5 to find out how.
  • No: Great! Move on to question #4.

4. Measure the sending unit connector’s electrical resistance. Is the reading within the minimum and maximum specs?

Disconnect the sending unit connector and use an ohmmeter to find out its electrical resistance. For a more accurate reading, drain the fuel tank, note the reading, and then fill it up and note the reading again. Doing this will help you figure out whether the sending unit responds appropriately. Your owner’s manual should give you the minimum and maximum specs.

  • Yes: The sending unit works fine. The issue is likely the fuel gauge or the wiring. Move on to question #5 if you haven’t checked the wiring yet or jump ahead to question #7 if you’ve already confirmed that the wiring is okay.
  • No: Congratulations. You’ve found the problem! The sending unit is bad and needs to be replaced.

5. Check the fuel gauge wiring. Do you see any physical damage?

Such examples include loose or corroded wiring terminals, shorts in the wiring, or frayed or open wiring insulation.

  • Yes: You might’ve found the culprit: damaged wiring. The best thing to do is replace the bad wire(s). Even if this doesn’t fix the problem, it’s still good that you replaced the wiring anyway because doing so prevents any problems that might crop up in the future.
  • No: Then this leaves the fuel gauge (and the sending unit if you haven’t tested it already). Move on to question #6.

6. Have you tested the sending unit yet?

  • Yes: So the sending unit is okay and the fuel gauge is the issue. Move on to question #7.
  • No: Go back to question #4 to test the sending unit.

7. Get various resistance readings from your fuel gauge. Does the gauge respond when you change the resistance?

To do this, you have to simulate input from the sending unit by using a test box or resistor jumper wires.

  • Yes: Then it’s probably just a wiring issue in the gauge. Move on to question #8 to figure out what it is.
  • No: You’ve found the likely problem: a broken gauge! Replacing it will do the trick.

8. Use an ohmmeter to check the gauge’s internal resistance. Is the resistance between 10 and 15 ohms?

  • Yes: Well, then we’re stumped! Everything seems to be working fine and we're not sure what's making your fuel gauge go haywire. A qualified mechanic may be able to figure out what's going on.
  • No: If there’s no resistance, then the gauge has a short. If there’s high resistance, then the gauge has an open. You can either replace the wires or get a new gauge.