It sounds like your Jeep Wrangler failed the LPFET (Low Pressure Fuel Evaporative Test). The Low Pressure Fuel Evaporative Test (also known as the EVAP test) ensures that there are no leaks in the EVAP system, between the vapor hose leading from the gas tank to charcoal canister, including the gas cap.
Here is how the EVAP system is tested: The LPFET involves the smog technician clamping the end of the hose leading to the charcoal canister and pressurize the gas tank with Nitrous Oxide via the LPFET test tool. The tool will then calculate the drop in pressure due to any leaks. There should be little to no drop in pressure once the EVAP system (gas tank) is pressurized.
The fix involves locating the leak and repair the vapor hose, or component which is leaking. Depending on where the leak is located, this repair may be cheap or expensive, but the first step is to find where the leak is at. The gas cap test is conducted on a separate test machine which is attached to the smog check machine. The gas cap test is not part of the LPFET.
