Can I Pass a Smog Check If My Check Engine Light Is On?

February 15, 2012

Even if a car's check engine light is on for reasons that don't involve emission problems, testers will automatically fail the car.

Check Engine Light

We've all ignored our check engine light at one point or another, but that may not be a wise decision in the event of a smog check. If you take your car to a smog technician and the check engine light comes on during the test, the technician will immediately give your car a failure mark. Thus it is always important to see if this is working when you go to take your next test.

What Does a Check Engine Light Indicate?
The check engine light is there to let you know that there is something amiss with the engine or the drive train. There is probably some sort of malfunction that needs to be taken care of. You will fail a smog check with the light on because the technicians want to ensure you have such a problem looked at.

When the light is on, the engine emissions control system or unit may start to operate from pre-programmed information and not from real time. This could cause poor emissions, which in turn will make your car fail the smog test.

What Can You Do?
Even if you manage to get the check engine light to shut off by trying to erase your car's trouble codes, the likelihood of passing the smog test is extremely slim. The best thing to do is to take your car to your mechanic and get the car repairs taken care of.

Related Questions and Answers

How Do You Clean an O2 Sensor?

"How to clean an O2 sensor?", has a very straightforward answer. To clean an O2 sensor, and save a few dollars, you will need a proper sized set of wrenches, flathead screwdriver, pliers, and a Phillips screwdriver. If needed, a set of car jack stands. To clean the O2 sensors, located at the engine's bottom, drive your car onto the jack stands and chock the rear wheels. Then, slip under the car and trace the assembly so you'll know how it goes back together. Then, disconnecting the wiring harness, slip the O2 sensor out of its holder and gently clean it with a small amount of gasoline (a good solvent) on a rag. Dry it, or use a very fine brush and burnish it until the dirt is gone. Reassemble it and you are finished.

What Does the O2 Sensor Do?

You might wonder what does the O2 sensor do, as there are usually two O2 sensors. One on the intake side, and one on the output side. The output sensor manages the car's microcomputer so that the emissions pattern remains within established federal parameters. While the intake O2 sensor responds to the demands of the output sensor in adjusting the stoichiometry of your vehicle. With today's lean-running, hotter burning ignition systems, your car is supposed to be putting out a minimum of CO2 emissions, NOx emissions and CO emissions. Properly operating O2 sensors ensure that stoichiometry (14.7:1 fuel-air) balance, so your car's emissions remain clean and within federal guidelines. A secondary reason for the O2 sensor function is to ensure your car meets the MPG mandate for the model year.

How Much Does an O2 Sensor Cost Brand New?

"How much does an o2 sensor cost?", is the question which doesn't have a specific answer. The cost varies from $40 to $500, depending on whether you change two sensors and their harnesses. This is probably the best way to proceed. By changing the sensors and harnesses in tandem, you are trying to ensure everything works correctly. Individual costs range from $40 to $80 per sensor. If you choose to replace the wiring harness and sensor, your costs will vary from $250 to $500 for both units. The choice is yours. Although changing both units is probably the best way to go to ensure maximum performance. The importance of the O2 sensor cannot be underrated, as it is the key to your car's performance. One note, if the "check engine" light comes on and your car is running rough, chances are the O2 sensors need replacement.

What is the Best Method for How to Test an O2 Sensor?

Since most drivers don't have access to auto emission testing equipment, here is how to test an O2 sensor: watch the idle. If your car suddenly starts to have a rough idle at a stop light or stop sign, it probably indicates the oxygen sensor is failing. It's because the feedback it is giving your system is faulty. It may try to lean out the fuel/air mix, when it should be somewhat richer, and vice versa. Another sure-fire test is your gas mileage. If the mileage goes down drastically, it is likely the O2 sensor has failed. The final exam though, is your state's emission test that will give you ultimate proof.

How Many Types of Auto Oxygen Sensors are there?

There are two types of auto oxygen sensors used in cars and trucks today; heated and unheated. Since oxygen sensors work most efficiently when they are heated, the auto industry has been moving toward the four-wire heated Oxygen Sensors so engines can work more efficiently, more quickly. The second type of sensor, unheated, is a two-wire sensor that relies on engine heat to work efficiently. You will find heated oxygen sensors used in the intake and exhaust manifolds. You may also find a pair of them used on the catalytic converter, checking the products that your engine is putting out. These are the sensors that ultimately indicate the health of your emissions system.