• Not having a good common ground for both gauge and sender, often caused by the fuel level sender not having a good ground.
• Not having a good signal to the gauge.

Step 2: Verify the sender is working
Disconnect the fuel level signal wire and measure the ohms on the signal wire with an ohm meter (multi meter). If possible confirm the ohms signal near empty and near full.

If you do not get a good reading your sender may be bad and require replacing.

Step 3: Confirm the fuel level gauge is good
Option 1: Check by Grounding the Signal
Hook up power and ground to the gauge but do not connect anything to the signal connector (S). On a 0-90 ohm fuel level gauge with no signal and power on the pointer should go all the way to the left. Now add a ground connection to the S terminal (use a jumper wire between G and S terminals) while keeping the power (I) and ground (G) connections in place. With a grounded signal connection (0 ohms input) the pointer on a 0-90 ohm gauge should move up towards the E marker.

Option 2: Check with a Resister
Another way to verify the gauge is working correctly is to "fake" a signal by putting a resistor across the S (signal) and G (ground) terminals. A 47 ohm resistor is ideal. On a 0-90 ohm fuel level gauge a 47 ohm "signal" will move the pointer to approx 1/2. The pointer will move to a different position for other ranges of fuel level gauges (i.e. 73-10 ohm, 20-150 ohm).

If you have a resister of a different value it will move the pointer to the corresponding dial position for your specific fuel level gauge. For example, if you use a 100 ohm resister on a 0-90 ohm fuel level gauge the pointer will go to Full +.

Option 3: Check with Another Known Good Signal
If you don't have access to a resister you can use the signal from your oil pressure gauge to test the fuel level gauge. Connect the power, ground and signal from the oil pressure gauge and connect them to the fuel level gauge.

If you have a 0-90 ohm fuel level gauge the pointer should move to a similar position as the pointer did on the oil pressure gauge. If you have another range fuel level sender the pointer will move to a different location but should still move. If the fuel level gauge responds with the oil pressure connections you know the gauge is good.