A clean engine bay makes leaks and wear visible, which is exactly why inspectors look there. It is also where careless cleaning turns a cosmetic job into a diagnostic one.
What goes wrong
Pressure washing forces water past seals that were designed to shed it, not resist it. Connectors, the alternator, intake components and exposed sensors are all vulnerable, and the resulting fault often appears days later, which makes it hard to connect back to the wash.
The safe method
Cover sensitive components before any liquid appears. Work on a cool engine, never a hot one. Use a dedicated degreaser applied by hand, agitate with brushes, and rinse in controlled volume rather than under pressure. Dry with compressed air where you can reach.
Finishing
Dress surfaces to a matte, factory-correct appearance. Glossy silicone dressings look impressive for a week, then attract dust and migrate onto belts and pulleys. A clean bay should read as well maintained, not as recently sprayed.