fuel pressure regulator, and the gauge says 0, then you don't have to worry about fuel or pressurized air spraying you in the face when you remove the fill cap for the fuel tank. The pressure is 0 or very minimal, and the system is normal. The fpr regulates the pressure upstream of where it's installed. Kind of like putting your thumb on a garden hose to build pressure in the hose. Some of the water shoots out, but the pressure is 0 when it gets past your thumb.