Smog checks are required in most of California biennially, or every two years. Even if you move from another state with a vehicle that has a valid smog certificate, you will need to get a California inspection before registering the vehicle in the state. This must be performed within 20 days of establishing residency. 
It’s not necessary to get an inspection for certain older cars because they’re exempt because of a grandfathering clause. Older vehicles also have less rigid requirements to pass the inspection based on the make, year and model. Vehicle exemptions to the biennial smog check include:
- Natural-gas powered vehicles over 14,000 pounds
- Diesel-powered vehicles that are 1997 or older models or weigh more than 14,000 pounds
- Gas-powered vehicles that are 1975 or older models
- Electric vehicles
Diesel vehicles don’t qualify for the new-car smog check exemption. The following California counties require smog checks only in certain ZIP codes:
- El Dorado
- San Bernardino
- San Diego
You must provide a valid smog certificate when you sell your vehicle in California. The certificate must be dated within 90 days of the sale. New vehicles don’t require a smog check for six years, which is added to the model year. For example, a 2012 vehicle wouldn’t need an inspection until 2018. However, if you decide to sell your newer vehicle, you’ll need to provide a smog certificate after four years.
What a Smog Check Measures
Smog checks measure the level of pollutants that the combustion of fuel in your vehicle generates. Your vehicle is tested on a dynamometer to simulate moderate driving conditions. Everything is analyzed by computer and automated so that your technician has no control over whether a vehicle passes or fails the emissions part of the test.
The California smog check is used to measure the levels of hydrocarbons, or HC, and carbon monoxide, or CO, in your exhaust. If you regularly maintain your vehicle and respond when your engine light comes on, your vehicle will likely pass. You should prepare for the smog test by checking a few key conditions and systems. Elements of your California smog check include the following: 
- Self-diagnostic reports from late-model vehicles that have an OBD-II, or on-board diagnostics system
- Visual and functional tests conducted by your technician that include visual inspections of your tailpipe, emission control components and automotive systems
- Functional tests of exhaust gas, recirculation system, fuel evaporative system, engine light, ignition timing and gas cap fit
- Emission standards that include evaporative emissions and exhaust standards that vary by your vehicle’s weight, model year, make and model
- OBD-II inspections and self-diagnostic reports of 1996 and later model vehicles
Scheduling Your Test
You can schedule your smog check at a Test-Only station, Test-and-Repair Station or Repair-Only station. Some stations offer pre-testing checks that don’t get forwarded to the DMV. Some repair and testing stations are also STAR stations where performance standards are more closely monitored. Although some vehicles are chosen at random, others are chosen for mandatory STAR checks because they’re more likely to emit higher levels of pollutants.
Older model cars can pass with less stringent standards, but emission standards are regularly revised. The best strategy for passing your smog check is to perform regular vehicle maintenance chores as recommended and to avoid tampering with the emissions control system of your vehicle.
Unfortunately, you can’t find a “liberal” technician that will go easy on the inspection. The testing process is totally computerized, so your vehicle must meet the standards on the day that it’s inspected. If you don’t want to face the expensive repairs that might be immediately required, the following best practices can help you pass a smog check:
Clear that Nagging “Check Engine” Light.
If your “Check Engine” light is lit, you’ll automatically fail your smog check, so you need to find out what’s causing the light to remain lit. The most common reason for the light is a faulty oxygen sensor. This runs around $168 when compared to the $1,000 cost of repairing a catalytic converter, which might become necessary if you drive with a faulty sensor for a long time. 
Run Your Vehicle at High Speed on the Highway
Running your car at highway speed within two weeks of your test accomplishes two goals: it heats up your catalytic converter so that it burns off contaminant deposits and it ensures that your car can reboot its OBD-II monitoring system if the battery has lost power recently.
Repair Any Moderate-to-Severe Leaks
Even if you pass the smog test, your technician won’t pass a vehicle if it has noticeable leaks of coolant, oil, transmission or brake fluids. Check for leaks before the smog check including all hoses and caps. Fluid leaks generate safety risks and generate potential engine problems.
Remove Excess Weight
Carrying more weight–such as heavy gear in the trunk, luggage carriers, trailer hitches and bicycle racks–causes your engine to work harder, so remove any nonessential weight until after the test.
Check Your Gas Cap
Poorly fitting gas caps allow unhealthy amounts of hydrocarbons to evaporate from vehicle gas tanks. A damaged or ill-fitting gas cap can release too many pollutants into the air. Evaporation can also damage your car’s performance and waste fuel, so it’s important to use a well-fitting cap. Your gas cap will be inspected during a smog check.
Change Your Oil, and Top Off Fluids
Dirty oil is full of hydrocarbons, which increases engine and emission contaminants. Fresh oil will increase your chances of passing the smog check. While getting an oil change, you should ask your technician to inspect the hoses to detect any cracked, broken or disconnected sections that might generate smog emissions.
Using a Fuel Additive
Older vehicles often benefit from a fuel additive to help clean clogged fuel injectors, which will help your vehicle burn cleaner. Some experts consider this unnecessary in California because the state claims to have the best gasoline in the world. Many additives are already in the gas and are chosen for the ability to keep your engine clean.
Battery Disconnection or Failure Resets the Monitoring Computer
If your battery was recently disconnected or required a jump-start, your car’s computer will erase its self-test monitoring history by resetting the OBD-II. Without this information, vehicles won’t pass the smog check. You need to drive between 100 and 200 miles in a week’s time to reset your emission monitors. You might need to take a quick trip before getting a smog test to ensure that your system is online.
Miscellaneous Tune-Up Tasks Increase the Likelihood of Passing
It’s important to ensure that your vehicle is running at maximum efficiency if you want to pass the smog test. There are some key maintenance practices that might increase your chances of passing by reducing the strain on your engine. These include the following:
Adjusting Tire Pressure
- Adjusting Tire Pressure Inflate your tires to the maximum limit that’s recommended by the manufacturer. Vehicles are checked on a dynamometer to simulate typical low- and mid-speed driving situations. Tires inflated to the highest recommended pressure generate less friction, which results in less strain on the engine.
Get a Fresh Tank of Gas
- Get a Fresh Tank of Gas It’s best to purge your tank of old gas before testing. Refill the tank with the high-octane gas available at a busy station so you don’t have to worry about getting old, contaminated gas. High-octane gas burns faster and hotter, which causes leaner combustion with fewer contaminants.
Drive at Highway Speed Before the Test
- Drive at Highway Speed Before the Test Driving your car at highway speed can help to heat your catalytic converter enough to burn off gas and oil residues. It’s also beneficial to run your vehicle on the highway before the test to raise your operating temperature. This makes your engine perform more efficiently, and just running the engine before the test to warm it up is insufficient. Getting the temperature higher also serves to open the choke for greater mixing efficiency of the fuel and air components of combustion.
Get a Tune-Up
- Get a Tune-Up Getting regular tune-ups is important for many reasons that include safety, fuel efficiency and better vehicle performance. Getting a tune-up right before your smog test will uncover any problems such as misfiring spark plugs, poor timing, bad points, carburetor or fuel injection problems and filters that need changing. Don’t get a tune-up immediately before the test, however, because you need to run the vehicle 100-200 miles to reset the emissions monitoring self-test feature.
Pre-Inspections Give You Extra Time
If you can’t immediately afford the cash or time to repair your vehicle after a failed smog check, you can find many facilities that are willing to conduct a pre-inspection check without recording the results officially. Of course, it’s necessary to do this well before your deadline expires so that you’ll have time to budget for any essential repair work.
Other “Home Remedies” Might Cost More than Fixing Smog-Related Problems
There are many sketchy or risky ways to pass a smog test, but they are usually not worth the risk. There are also ambiguous claims that tend to balance out under real-world conditions. For example, some people claim that rainy weather can cause tires to slip when your vehicle is on the dynamometer. However, the warm-up cycle usually dries the tires sufficiently so that moisture is seldom a problem. Increased humidity can actually lower the emissions of certain pollutants.
Some “experts” recommend using all your gas or draining your tank and adding a small amount of ethanol before the test. Ethanol burns hotter and cleaner, but you run the risk of overheating your engine. The costs of this kind of damage far exceed what it would cost to repair your emissions system, and you would need to drain your tank again or fill your tank to dilute the ethanol after the test.
You need a valid smog check certificate to sell your vehicle, and the test must be done within 90 days of the sale. It’s important to consider this timetable when buying, selling or trading a vehicle in California. Above all, remember that it’s in your best interest to keep your vehicle tuned-up and in good repair for safety reasons, best performance and resale value. Proactive maintenance also prevents expensive repairs and damage to parts that might require replacing costly original equipment.
 Dmv.org: California Smog Check
 Bar.ca: Q&As Smog Check Program
 Edmunds.com: How To Pass a Smog Check on Edmunds.com