Make sure your cars "Check Engine" light (also called the Malfunction Indicator Lamp - MIL) is not on and that it has not recently been fixed, when you go for the smog test. The reason is that during your smog test the technician will read the OBD II (On Board Diagnostics) data from your car to understand any problems that might impact the emissions from your car. Every time the "Check Engine" light comes up due to any issue there is an associated error code logged in to your OBD II system. Now if you’ve recently flipped the switch or disconnected the battery of your car although the MIL might clear off but your might have not set your cars readiness flags, which might be a reason your car will fail the emissions test. More on this in this FAQs section.
Get your smog test a couple of days after an oil change. This will ensure that your car atleast has fresh oil and oil filter. However, don’t get the smog check done right after the oil change. The reason is during your oil change most technicians will adjust your MIL. See point above. Tell them not to disconnect your battery for any reason and ask them of the OBD II code if you MIL is on. It is advisable to fix the problem due to which your MIL was on.
Don’t take a car that has been sitting around for a couple of months for a smog test. This is because the gasoline in that car is stale. If you have ever stored away your lawnmower for winter and then tried to turn it back on following summer, then you know what I’m talking about. Gasoline has a shelf life of about 60-90 days. Older gasoline, even when usable, will burn with higher emissions and you might fail the smog test just on account of that. So it’s advisable to get a new fill of gas from a busy gas station right before your test.
Make sure your gas cap is okay and it makes a clicking sound when completely tightened. You can ask the technician this before you start the test. It’s easy to check. Some smog test stations will sell you a new gas cap if that’s a problem.