Check and or replace the O2 sensors.
Drain transmission fluid and change filter.
Replace cabin air filter.
Inspect and replace PCV valve.
Inspect the EVAP system and repair as needed.
Check the EGR system and repair as needed.
Service cooling system.
Check and or replace spark plugs as needed.
Check and or replace plug wires as needed.
Check and or replace Coils as needed.
Replace the Fuel Filter.
Replace the Air Filter.
Check tires for wear.
Check belts and hoses and replace if worn or damaged.
Check drive axle boots for damage and replace as necessary.
Check brakes and replace pads or rotors as required.
Inspect suspension and steering components.
Inspect the exhaust system for damage or leaks.
And most importantly...
Please add your year and trim of your vehicle to your signature line. That way when you start asking questions in other areas of the forum, we all have a better idea on how to assist you. Click on "Quick Links" above, go down to "User Control Panel" and then click on "Edit your Signature" It'* pretty straight forward from there.