Below are some information to prepare for a successful smog test if you have an older car.
If your car displays a “Check Engine” light, that’s an automatic smog check failure. You’ll need to get a diagnosis and fix before you test.
Dirty oil in the crankcase could release additional pollutants, which could cause the car to fail the smog test.. While the mechanic is changing the oil, ask him to do a visual inspection of the car’s engine to see if any hoses are cracked, broken or disconnected.
Many states require a dynamometer test, which positions the car’s tires on rollers that allow the engine to run at high speeds while it is stationary. If the tires are under-inflated, the car’s engine works harder to achieve the engine revolutions required by the test.
Have any required maintenance performed well before the smog test, Most mechanics disconnect the battery while doing a tune-up and this resets the car’s onboard computer. The car then needs two weeks of driving to run all the diagnostic tests needed to pass the smog test.
In some states, smog check stations will do a less expensive pre-inspection that shows if a car will pass or fail without officially recording the results with the state’s registry of motor vehicles.
Since the test runs the car at high speed while it is stationary, less air flows through the radiator to cool it. So make sure to fill your coolant tank properly. Also, the car will be on a dynamometer, possibly at an angle. If gas is low in the tank, this could expose the fuel pump and put vapor in the fuel line, causing the car to fail the test.
This gets the catalytic converter hot enough to burn out any oil and gas residues.