How to pass a smog test — and what to do if your car fails

When my car failed California’s smog test last year, I wandered from one garage to another trying to determine how to bring it into compliance. After speaking with several automotive experts and popping for a few relatively inexpensive repairs, I finally got my new smog certificate.

But with what I know now, I might have passed in the first place — or at least fixed my 1997 BMW 328i with less hassle. Here’s my hard-earned advice so your car can, hopefully, pass with flying colors.

1. Be proactive

As soon as you get a registration renewal notice calling for a smog check, mark your calendar and begin preparing.

“That way, if something goes wrong, you have plenty of time to address your issues,” says Dave Skaien, manager of the Automobile Club of Southern California’s approved auto repair program. Skaien also recommends paying registration fees early to avoid any late charges, although you won’t get your new registration until the smog test is complete.

Most states require emissions tests every two years, or before selling a car, but costs, requirements and testing methods vary. For example, some stations use a dynamometer test, particularly for older cars, which allows the wheels to turn and simulate driving conditions. So check your notice to see whether you need to bring your vehicle to a specific type of station.

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2. Perform necessary maintenance

Your car owner’s manual outlines required maintenance at different mileage intervals. Tackle any necessary service items before having your car tested.

But don’t just wait for a smog test to do routine maintenance, Skaien says. Regularly perform oil changes and tuneups and be sure your tires are properly inflated. Then, when a smog test is required, you’ll be ready.

You can also try using a fuel injector cleaner, which can help clean carbon deposits from your engine and reduce emissions.

My car failed in part because I’d neglected to change the spark plugs, part of my routine maintenance, which was a $299 job. Unfortunately, that solved only part of my emissions problem.

3. Clear your check engine light

If your check engine light is on, you have a problem you need to address immediately, says David Rich, technical director for CarMD, an automotive repair information site.

Problems that trigger the check engine light could require inexpensive fixes, like a loose or damaged gas cap, or you might have to spring for more serious repairs. Find out and fix what’s causing your check engine light to illuminate.

But even after repairs that clear the check engine light, you need to drive until the car’s computer verifies that all operating systems are working correctly. That’s why the next step is so helpful.

4. Precheck your car

AutoZone stores, for example, offer a free diagnostic analysis you can use to pretest your car for smog-check readiness, Rich says. These tests aren’t just for smog tests, but they’ll alert you to any remaining issues. This is especially helpful if you’ve just had repairs performed or if you have an older car you’re worried might fail.

Tips for test day

Warm up your car. About 15 to 20 minutes of “reasonably enthusiastic driving” will bring your catalytic converter, which burns emissions, up to operating temperature, Rich says.

Remove any junk in the trunk. Extra weight makes it harder for your car to pass, Skaien says. So take your golf clubs and bowling balls out of your trunk.

Avoid rainy days. Wet tires can slip on a dynamometer test and ultimately cause your car to fail.

If you fail your smog test

Knowing how to fix a car that’s failed a smog test is harder, but I have some tips for that, too.

The smog technician usually provides a form showing the reason your car failed and its emissions levels. Bring this to your mechanic to find out if they know how to fix the problem.

If you still have trouble locating the issue, ask your mechanic to perform a “smoke test” which forces smoke into the car’s evaporative system to find any leaks in the rubber hoses leading to and from the engine. This is how I finally located the cracked rubber hose that caused my car to fail. The test and repair cost $139.

For cars that fail the first time, Skaien recommends retesting it at a “fix and repair” station rather than one that does testing only. Technicians there are experts in emissions systems repairs, minimizing the need for all of the driving back and forth between garages and testing stations that I experienced.

Finally, if you fail the test and you’re low on money, you might be eligible for financial assistance. For example, the Consumer Assistance Program offers up to $500 to qualifying car owners for emissions-related repairs.

This article was written by NerdWallet. Philip Reed is an automotive expert who writes a syndicated column for NerdWallet.