The Smog Check

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If you don’t have mandatory emission control (smog) checks in your state, you will soon. The pollution caused by the daily emission of tons of hydrocarbons, carbon monoxide and oxides of nitrogen into our atmosphere is a phenomenon no longer limited to California, which has the most stringent emission control policies in the country. Real hard facts of acid rain, the greenhouse effect and the destruction of our ozone are prompting the federal government to get involved. Many states have already adopted California’s air quality control policies.

So the smog check is here to stay, and because it's a mandatory repair service, it's prone to dishonesty. Smog checks have not been around for a very long time, but there have been more lawsuits and more complaints filed with the California Bureau of Automotive Re pair (BAR) with this service (per ratio) than any other.

Because California has the most stringent emission control procedures and most states are adopting these procedures, most of what I’ll talk about applies to this state. Some of the procedures and legalities may differ where you live, but you’ll get the basic idea of what’s involved and what’s not involved, in a basic smog check.


There are three major types of exhaust gas pollutants given off by the internal combustion engine: hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).

It is the function of the various emission control devices installed on your vehicle to reduce the amount of HC, CO, and NOx emissions reaching the atmosphere as much as possible. Emission control devices can do this by limiting their formation during combustion, or by neutralizing them after they have formed. For example, spark retard and EGR (exhaust gas recirculation) systems help prevent the formation of CO and NOx to promote more complete combustion of HC.

Various “afterburner” devices such as the air injection system and the catalytic converter, work by burning up any residual HC and CO after they have formed in the combustion chamber but before they exit the exhaust.


In California, a car must be smog checked every other year when it's time to be re-registered. Owners are required to submit proof of a smog check before they receive their tags, unless the car is deemed exempt, or the cost of repairs exceeds a preset limit. Smog inspections are also required whenever there is a transfer of ownership with a vehicle.


If you’ve recently swapped or replaced your engine, you are required to tell the mechanic this before the smog check is performed. He is supposed to refer you to the nearest California Bureau of Automotive Repair office for authorization and referee intervention. You’ll have to bring all of your paper work to verify the source of the new engine and date of installation. The referee station then has the option of “approving” or “disproving” the engine, making sure it compiles with the original manufacturer’s emission control components and operability. If it fails, they’ll tell you what needs to be done to bring it up to specs. Once you do that, it can be retested.

Initial Inspection—Smog check stations are so designated by the state and issued a license after complying with tests and regulations. When you first arrive for the smog check, your car is given a visual inspection just to make sure all emission devices are still on the car and secured properly. This inspection is prior to the actual test, before any analysis or adjustments are made. If anything is missing, such as belts, or vacuum hoses, you’ll be told that the condition or problem has to be fixed before the test. And, of course, they’d be willing to fix it for you. Following are some of the emission control systems and components for which a licensed mechanic will visually check to determine if they have been disconnected, missing, or modified:

  • 1. Positive Crankcase Ventilation (PCV)
  • 2. Thermostatic Air Cleaner (TAC)
  • 3. Air Injection (Al)
  • 4. Fuel Evaporation System (FE)
  • 5. Fill pipe Restrictor (Gas Nozzle)
  • 6. Catalytic Converter (Oxidizing Catalyst)
  • 7. Three-Way Catalytic Converter (TWC)
  • 8. Exhaust Gas Recirculation (EGR)
  • 9. Spark Advance Controls
  • 10. Computer Controlled System (CCS)
  • 11. Fuel Injection Controls
  • 12. Retrofit Devices

Next the mechanic will enter the year model of your car, its engine size and number of cylinders into a test analyzer. For 1972 and newer vehicles, he will look at the under-hood emission control label. Both California and federal laws require that every new vehicle have a permanent label in the engine compartment containing: the name of manufacturer; vehicle conformity (U.S. EPA) emission control requirements; engine size in cubic inches, liters or cc’s; emission control type; engine tune-up specifications; and adjustments recommended by the manufacturer. This label is commonly located over the radiator in many cars, or on the fender side panel.

If for some reason the label is missing, obliterated, or difficult to read, the mechanic can refer to an emission control system application manual that will have this information along with the necessary diagrams.

Analysis—Once a vehicle has been visually inspected and has proven to the licensed mechanic that all systems are intact, the next procedure will be to attach the vehicle to an emission exhaust analyzer to test each system for operability and overall function.

This requires the mechanic to insert an exhaust probe, commonly referred to as a “sniffer,” into the tailpipe of the vehicle. The sensing device located in the probe and the line that's attached to it relay the condition of the exhaust gases to an exhaust gas analyzer, which determines the precise amounts of chemical emissions from the exhaust and translates this information to the mechanic via a printed readout and/or digital display.

On the readout, emission control components will be listed along with the subsequent reports on those systems. The systems will be marked with either a “pass” or “fail.” A “fail” response indicates that a system is malfunctioning, disconnected, or in need of repair. Or that the overall emission gases exceeded the maximum allowable amounts for that year and make automobile.

A percentage reading will accompany the emission results. Here is an example of a typical computer tape readout.

No doubt the test information seems confusing. Under the emission results heading you will notice that this car produced a reading of CO (carbon monoxide) in an amount of 1.56. After the 1.56 reading there is an “F” which denotes a fail. The standard for that car for CO emissions is 1.50% or less, as indicated by the number to the left. Naturally, the car flunked the first smog test. At the very end of the printed readout there is a notation to “RETAIN THIS COPY FOR USE ON RETEST.” In other words this car will have to be retested, but first the (idle) emission under the CO heading will have to be adjusted or repaired (In this case an adjustment will have to be made to bring the vehicle within acceptable specifications.)

Retest—When the vehicle is tested again it will receive another printout and it will not be unlike the first one aside from some different phrases and the addition of a “REPAIRS PERFORMED” section that will end at the bottom of the list. On the new printout (since it passed this time) will appear the words PASS under the “RESULTS OF YOUR INSPECTION” heading. Beneath that heading will appear “CERTIFI CATE OF COMPLIANCE: B4389834” with the positive phrase, “CONGRATULATIONS YOUR VEHICLE PASSED THE CALIFORNIA EMISSIONS INSPECTION.” Thanks a lot.

Cost—In California, for instance, this first inspection costs around $20 for the test, plus an additional $6 or so for the certificate (paper work). If our test car had passed the first time, the cost would be around $26 with no additional expense. However, this vehicle failed and had to be repaired/adjusted and then run through the test again.

In California there is a maximum fee for making this and most other adjustments and light repairs to your emission control system. There is a $50 maximum charge for such a second testing and repair! adjustment, which includes the first test fee, which in this case was $26. That means that the mechanic could charge up to $24 more to reach the maximum. And this is where it gets a little bit tricky, especially if you fail and need some type of repair/adjustment.

The cost for the certification and smog check varies from state to state. You will have to check with your local certified smog test and repair station.

Passing Grades—Frequently, vehicles that have just had tune-ups prior to a smog inspection pass the tests regularly. If you are ready for a smog inspection, a tune-up plus the replacement of all filters will help your car pass the test. Also, make sure all vacuum lines are routed and tightly fitted to their proper connections. Consult your emission control diagram for proper hook-ups and routing. You can find it in your owner’s manual, or the sticker will be prominently displayed in your engine compartment.

Recertification—Once a vehicle has passed a smog check from a certain facility, the vehicle owner would be very wise to take that car back to the same facility for any future smog checks. Bringing along the past receipts and smog check printouts is an excellent way for the vehicle owner to show that his car has performed well in the past and has been certified with passing grades. It is a very good time to compare printouts and determine what systems on the vehicle have changed or are beginning to wear.


Quite often, you’ll see advertisements reading something like “Complete Smog Check $19.95” or my personal favorite, “Pass or Don’t Pay.” If your car can’t be fixed for $50 or less in order for it to pass the test, you will still get a smog certificate. But believe me, unless the problem is major, mechanics will do everything to tune the car to the $50 limit. Then your ticket will run $50, plus the $19.95 and usually $5 for the certificate. However, there are some facilities that advertise “Pass or Don’t Pay” that only offer a smog check—period. They’ll check your car but they don’t repair them or make any adjustments. If you fail the test, you don’t owe them a dime. If you pass, you must pay their price. However, they don’t work on the car: they are literally a check-only type of business. What you could do is have them run the check, and if it fails, go somewhere else you trust to have your car tuned, then go back for the test.

At least the law is working for you. Every test performed in California is documented, approved by an inspector, logged and turned over to the state for review by the Bureau of Automotive Repair, and other states have similar checks and balances. And the Bureau is always on the lookout for a falsified entry, inaccurate tape, or forged invoice, but they are understaffed and overworked. So if the mechanic does pull a sham he stands a good chance of getting away with it. If he is caught, he could then lose his license. But that threat doesn’t stop the dishonest mechanic from employing a few tricks.


A mechanic can skirt the legal boundaries of a smog check by just selling you parts ahead of time, telling you that the installation of these parts will help your car pass. Quite often he will do this just after the visual inspection but before the initial test. You could be sitting in the waiting room and suddenly the mechanic appears with your invoice. He explains to you that your air cleaner, PCV valve and filter, and a couple of vacuum lines need replacement. On the invoice you might find it written under the heading of “mechanic’s recommendations.”

Bear in mind now that he hasn’t even run your car through the initial test, but has in fact given you some immediate recommendations. All too often customers think that these preliminary recommendations are part of or essential to the actual test when they don’t really have anything to do with it, since he hasn’t tested your car yet.

If you query him as to why you need the parts he might say, “these parts are a good insurance before a smog test.” Or “the parts don’t look too good and new ones would help the test.”

He hasn’t said that you will actually fail if you don’t follow his recommendations. What he has done is raise a question, provide you with an unanswered mystery: “It just might fail without the part.” He could say, “I just don’t know about that EGR valve.” Whereupon you answer, “Do you think that I should replace it?” His answer will almost always be, “Yes, it’s best.”

If your car had been smogged, he would have had a tape readout in his hand to show you whether it failed or passed. That would be the proper time to buy the parts or recommended services to correct the failure and insure your certification—when he showed you a failed readout. If you’ve recently had your car tuned up, then I would be especially suspicious if any of the above happened.


Be most careful during the visual inspection prior to the smog check. A mechanic could insure a “max” on your in voice with a couple of nasty tricks. He could, while doing the visual, disconnect a vacuum line, or turn an air/fuel mixture screw out, which would immediately set the car up for a “fall” or “repair/adjust” reading, as soon as the initial test is performed. He could temporarily disconnect an emission control component with the deftness of Houdini and if you were not around to watch the operation, you would be none the wiser. I’ve heard of one mechanic who always kept a clogged PVC valve in his box, and when doing an inspection, ho would routinely substitute it for the customer’s part, which prompted a fail every time. To repair it, he would reinstall the customer’s original functioning PCV valve back onto the system and charge them for a new one nevertheless. Watch the mechanic closely. He should not have to lean into your engine compartment and handle anything, unless it's to test the tautness of a vacuum line. He is justified in touching something when he begins the actual test on the vehicle.


It’s happened before that a mechanic has deliberately set the air/fuel mixture to an overly rich condition during the visual inspection prior to testing the car. One of the best things you can do to protect yourself against this is to observe the mechanic during the visual inspection to make sure he makes no adjustments to the car. He shouldn’t even touch it until after he’s run the test, especially with a screwdriver.


In order for a smog test to be accurate, the engine must be up to operating temperature. A test given with a cold engine will almost surely fail, because most cars compensate for cold starting by going to an overly rich condition, also known as the choke setting. All other emissions control equipment on the car needs to be up to a specific temperature to function properly. If your car has been sitting for a long time, make sure the mechanic warms it up sufficiently before performing the test. This time period could be as long as 30 minutes.


If your car is still covered under its basic warranty, and it fails a smog check, then you should take it back to the dealer. Federal laws require that carmakers ensure their cars are emissions-legal for a period of 5 years. If you aren’t aware of this fact, the smog-test mechanic probably won’t tell you. For more on warranties, go to the end of section 11.

  • The three major types of exhaust gas pollutants given off by the internal combustion engine are: hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).
  • It is the function of the various emission control devices installed on your car to reduce HC, CO, and NOx emissions. You should maintain your factory installed emission control components for this reason.
  • Smog checks; repair/adjust, and certification can only be done by an officially licensed smog check repair station with a licensed and authorized mechanic per forming the adjustments and repairs.
  • For 1972 and newer vehicles, the emission control label is located under-the-hood and displayed in plain sight. If it's not, or has been obliterated, a mechanic can refer to an emission control system application manual.
  • If a mechanic tries to sell you parts after a visual inspection but before a test, tell him to perform the routine check anyway because the tape readout will be the judge of that. A mechanic can't tamper with the results of the machine; you are assured a fair shake with the analysis.
  • Make sure your vehicle is brought to full operating temperature before actual testing begins. Cold, or choked engines often fail the smog check. Tell the mechanic to warm it up.
  • In California, there is a maximum $50.00 cost limit for fixing a failed car. It applies to: adjustments/repairs to lower tailpipe emissions; correction of conditions causing “check engine” lights to be on; repairs to worn-out or defective emission controls that are not missing, modified, or disconnected. It does not apply to a major or minor tune-up.
  • Is there any way to get a certificate if the emissions still fail the standards after the repair and retest? Yes, if the originally failed emission is reduced and the other emission is still within standards. Or if further repairs will exceed $50.00, then a special certificate can be issued due to exceeding the cost limit. In California, if the emissions can't be reduced, the vehicle must be referred to the Bureau of Automotive Repair for review. The BAR will retest the vehicle.
  • A mechanic can refuse to inspect/repair or certify your vehicle if he finds: an obvious oil, transmission, fuel or coolant system leak; if the maintenance warning light is on; a safety problem; an excessive exhaust system leak; an inaccessible exhaust; high or low engine rpm; excessive engine or bearing noise; an unknown certification status. In these instances the problem area or system will have to be repaired by the vehicle owner before a test and certification can be given.
  • If you own a newer vehicle and your test shows that you have failed, it's possible that a failed part on your car (responsible for the emission failure), is under warranty. In this case, you can ask the advice of the mechanic as to whether a warranty part has gone bad, leading to the failed condition. If so, you can take your vehicle to the new car dealership and have the part replaced, then return to the station for a retest.

Next: Protect Thyself -- What to Do When You’ve Been; Ripped Off; Tips to Save Money

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