Smog checks are necessary inspections that ensure the cars on the road are not emitting excessive pollution. The spread of smog checks from state to state (under varying names and schedules) has reduced the amount of air pollution around the country, so as annoying as it might be to get that smog check notice in the mail, it is a task that leads to good outcomes. If you know your car will need a smog check in the next few months, it's time to take care of a few things to help your car pass. That includes not doing some things that could ruin your chances.
Ignore Maintenance in the Weeks Before
Smog checks are done on regular schedules, so you should have plenty of notice. Even if you totally forget and don't know about the check until you get your registration notice, you'll still have several weeks. In those weeks, but before the final two weeks or so before the test, get routine maintenance done. Get the oil changed, ensure there are no dashboard lights coming on (if the "check engine" light is on, you're not going to pass — get that issue fixed), and have the engine inspected.
Do Maintenance in the Days Before
Once you hit the two-weeks-or-so mark before the test, don't do massive amounts of maintenance work on your car unless it is necessary. When you do things like change the oil, change other engine fluids, or change other parts in the engine or emissions system, you can throw the system off for a bit until it adjusts. Doing small things like ensuring the tires are properly inflated is fine, but try to leave the engine alone unless you've got an urgent issue.
Let the Car Sit
Those two weeks leading up to the test? Drive the car on the freeway. You want long stretches of relatively high-speed driving so that the catalytic converter is cleaned out. The dirtier the converter, the higher the chance that your car will fail. Don't rely on surface-street jaunts where you have to stop a lot. Drive that car on the freeway daily.
Cars are tough, and if you've kept the car in good shape, driven it at faster speeds, and had it checked out, you should do fine. And keep in mind that if something does go astray during the test, you get a chance to fix it.
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