The student pilot departed Hawthorne, CA at approximately 1000 PST on a solo cross-country flight to Porterville, CA and return. During the flight, the pilot became lost and landed at Visalia, CA. While there, he obtained fuel and directions to Porterville and departed. He arrived at Porterville at approximately 1420 PST, met a friend on a similar flight, and both aircraft departed at 1455 PST. They arrived at Lompoc, CA at 1715 PST where the first pilot decided to fly back to Santa Maria, CA to purchase fuel. After landing and purchasing fuel, the aircraft departed on a VFR flight plan at approximately 1720 PST with Hawthorne, CA as the destination. At 1855 PST, the aircraft crashed approaching the threshold of runway 07 at the Ventura County Airport, Oxnard, CA. The toxicology report on the pilot showed a 48% carbon monoxide saturation in his blood containing 9.4% hemoglobin. The exhaust system was inspected and the left hand muffler showed signs of internal leakage into the cabin heater system.