This passing tip, as well as tips number 5 and 7 do not necessarily apply to your vehicle if it's 2000 model year or newer; this is not to say your vehicle's tires shouldn't be properly inflated, of course.
5. Change Oil - If it's close to your next oil change interval, go ahead and do it before the smog check. The PCV (positive crank ventilation) system of your vehicle is designed to allow your engine to breath fumes located in oil compartments (oil pan, ect.). The fumes are then burned through the combustion process. If the oil in your engine is contaminated due to inadequate oil changes it may very well cause your vehicle to fail the inspection.
Contaminated oils are high in Hydrocarbons (HC) and will present a rich mixture to the engine chambers. Avoiding oil changes not only causes pre-mature engine wear but can also cause your vehicle to fail the smog inspection. Change the engine oil.
6. Check Engine Light Off - Make sure your vehicle's Check Engine Light or Malfunction Indicator Lamp is not illuminated. A constantly illuminated Check Engine or MIL light is an automatic smog failure. A certified repair shop can diagnose the check engine light condition and offer you an evaluation and estimate. The home mechanic may also diagnose a Check Engine light malfunction via special Check Engine codes retrieval tools, available at most auto parts stores. Note: A smog technician can not refuse to test your vehicle based on the fact that he or she notices an illuminated light. The smog technician must perform the smog check and fail your vehicle. This is California State law. Be aware. Get a pre-test.
7. Battery Recently Disconnected? - If your vehicle recently needed a jump start or battery replacement we recommend holding off on getting the smog check for at least one week. Whenever a vehicle's computer (engine control unit) losses power, such as when it's battery has been disconnected or charge depleted, its internal self-test monitors are erased. Without these required Emission Monitors your vehicle can not pass the smog test. You will be required to drive between 100-200 miles and over the course of a week, called a Drive Cycle, in order to "complete" the emission monitors and be allowed to pass the smog check.
8. Avoid Rainy Weather - If your vehicle is model year 1999 or older it will be driven on a dynamometer during the smog test. When your car's tires are wet there is greater chance of slipping and losing traction as the smog technician tries to maintain required test speeds while on the dynamometer. Unstable engine and drivetrain torque is not the best conditioning your vehicle should receive if you plan on getting passing emissions test results. This is not to say you can't get a passing emissions test on a rainy day, certainly not. The smog machine has a "dry tires" mode, which will dry your car's, truck's, SUV's, or van's tires before the test. However as we mentioned, you don't want to place any extra strain on the engine before or during the emissions test.