Heavy smoke with a gas smell from exhaust
 

My son started his 2004 Mazda 3 the other day and noticed heavy white (dirty white) exhaust smoke from his tailpipe. We also noticed black colored liquid dripping from the pipe (smells like gas). The smoke does not smell like anti freeze so we are ruling out a blown head gasket and now focusing on injectors, valve guides and fuel injection system. The check engine light has been on for a week and had the codes read with them pointing to both the O2 sensors and throttle body. The engine sounds like it is idling normally (no abnormal noises) and seams to be running smoothly.

Would appreciate any suggestions on where to start first. Would like to start simple and not have to pull the head right away.