If you are investigating light-colored residue under the oil cap, save every stored and pending code before clearing anything. A scan may show temperature or misfire clues, but proper testing is needed to confirm coolant contamination. Record the engine temperature and load alongside the reading; the same value can mean different things in different conditions.
A basic reader may not access every module. Consider recent short trips and ambient temperature; a small cap deposit alone does not prove a head-gasket failure. Use the exact vehicle’s service information to interpret the result.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.