If your vehicle is showing fuel gauge level fluctuates wildly while driving, here are the most likely causes and the parts you'll need to fix each one.
The electrical resistor track on the sending unit wears out over time. As fuel sloshes and the float moves across these worn 'dead spots', the gauge receives erratic signals and jumps around.
If the fuel pump was recently replaced, the float arm may have been bent during installation, or the internal wiring connector wasn't fully seated, leading to erratic resistance readings.
The fuel gauge relies on precise electrical resistance. A loose ground wire or corroded connector at the fuel tank will cause the resistance to jump intermittently, making the gauge fluctuate.
Vehicles use a module or cluster logic to smooth out the fuel gauge reading so it doesn't move with every bump. If this logic fails, the gauge displays the raw, rapidly sloshing fuel level.
The fuel tank has internal walls (baffles) to prevent fuel from sloshing violently. If a baffle breaks loose, the fuel sloshes heavily while driving, physically throwing the float arm around faster than the system can dampen.