Station technicians see the same five failures every week, and none of them are the catastrophic catalytic converter failure drivers fear. Knowing the boring ones saves a return trip.
Loose gas caps top the list. The evaporative system pressurizes, and a cap that doesn't seal throws a code indistinguishable from a real leak. Tighten until it clicks, drive a few days, and the monitor resets itself.
Old oil is second. Blow-by gases circulate through cracked PCV hoses and contaminate the readings long before the engine runs badly enough for anyone to notice. An oil change two weeks before the test is cheaper than a retest.
Third: aftermarket cold-air intakes and removed resonators. Technicians must visually confirm the stock configuration, and a modified intake fails the inspection even when emissions are clean. Keep the original parts if you mod for sound.
A smog check station will usually tell you which of these tripped, but retest fees stack up. Five minutes under the hood with a flashlight beats a failed certificate every time.