How to Prepare Your Car to Pass an Emissions Test

truck with flowers in tailpipe

How to pass an emissions test (with OBD2 scanner)

The dreaded smog test – that annual or biennial check-up that your car must undergo in order to prove its roadworthiness.   In this article we’ll explain what the smog test is about, and how you can ensure that your car is in a condition to pass it. Read on to learn how to pass an emissions test , and the OBD2 scan tools that we recommend to pass an emissions test.

For this article, we’ll assume that your car was manufactured in the past 30 years with an emphasis on cars manufactured from 1996 or afterward as these comprise the vast majority of vehicles currently on the road an

Exhaust pollutants and how they are formed

Gasoline engines produce three types of pollutants, Hydrocarbons (HC), Carbon Monoxide (CO) and Nitrogen Oxides (NOx).   Diesel engines,  in addition to the above pollutants also produce ash or soot.   Hydrocarbons are un-burned, or partially burned fuel present in the exhaust.  They are formed by incomplete combustion.   Carbon Monoxide is also formed by the incomplete combustion, whereby a carbon atom only combines with a single Oxygen atom rather than a pair of Oxygen atoms to from Carbon Dioxide.    Unlike HC and CO, Nitrogen Oxides are not a product of incomplete combustion.  They are formed by the very hot combustion process itself or a lean fuel mixture.  Modern internal combustion engines try to limit both HC and CO production by optimizing combustion through careful control of the air and fuel mixture injected into the combustion chamber, and by optimizing the shape of the combustion chamber itself to ensure that combustion is as complete as possible.   NOx is controlled by various means, including not allowing the engine to run lean, and by cooling the combustion process through exhaust gas recirculation, whereby small amounts of exhaust gasses are re-introduced into the combustion chamber.    While careful engine design and engine management can reduce the amount of HC, CO, and NOx emitted, it can’t eliminate it.   To reduce pollutants even further, modern engines are fitted with catalytic converters which further reduce the amount of pollutants through oxidation and reduction …   However, for the catalytic converter to work efficiently, a perfectly matched air:fuel ratio (referred to as stoichiometric) must be maintained as much as possible (usually when the engine is running at a steady RPM).   

What is a smog test and how is it conducted?

Since it was recognized in the mid-1960s that the pollution produced by cars and trucks led to deteriorating air quality and impinged on peoples’ health and quality of life, governments have mandated increasingly stringent exhaust pollutant levels for new vehicles sold in the United States.   In the 1970s some States began requiring visual checks of vehicles to ensure that their pollution control systems were working correctly.   By the 1980s many states required vehicle exhaust emissions to be tested. Today about 30 states require periodic smog checks.

Every jurisdiction will have its own criteria for smog testing.  Generally it will consist of one of the two following types of tests.

  1. While the vehicle is running, the exhaust gases are measured to make sure that pollutants are within specified levels. 
  2. In the case of OBDII compliant vehicles, an OBDII scan is performed to ensure that the vehicle’s engine/emission control systems are performing within spec.  No physical exhaust pollutant measurements are taken.

Some jurisdictions may also include a visual test where the engine and emissions components are looked over to ascertain that nothing has been removed or disabled.

How do I make sure that my car passes its smog test?

No matter which style smog testing regime your car falls under, your first step is to check that the engine is working properly.   Modern engines rely on an array of sensors and other electronic components to ensure that they run as cleanly as possible at all times.   When one of these components fails, it may leave the Engine Control Unit (ECU) unable to calculate the correct amount fuel required resulting in sub-optimal operation and increased concentrations of pollutants in the exhaust.  In a modern car, any failure of a component of the engine management/emissions system will cause the Check Engine Light (CEL) to illuminate.

If your CEL is not illuminated and the smog test in your jurisdiction only requires a simple OBDII Scan with no physical test, congratulations, you’re ready for the smog test.  If your state requires an exhaust reading to be taken, chances are, your car will pass if the CEL is not illuminated, however you should use this opportunity to perform a basic tune up such as checking, and if necessary changing the engine oil and oil filter.  Since the late 1960s all cars and light trucks have been equipped with a Positive Crankcase Ventilation  (PCV) system which sucks fumes out of the engine crankcase into the air intake system.  Old, contaminated oil will be high in hydrocarbons which when burnt in the engine can increase exhaust pollutant levels.  Luckily, modern cars run cleaner and are built to much tighter tolerances than older vehicles, so if you keep to the recommended oil change interval and your car hasn’t experienced any engine problems, you are unlikely to have seriously contaminated oil.   

A severely worn engine can also fail the smog test as it allows oil past the piston rings into the combustion chamber.  Luckily, due to their sophisticated engine management systems, modern vehicles can travel vast distances without significant engine wear.   You are unlikely to encounter this problem.

If the CEL is illuminated the car will fail its smog test as its sole purpose is to notify of any problems that could affect vehicle emissions.  You can take the car to a repair centre to have the problem diagnosed and repaired.  Or, if you’re the adventurous type, read on as we’ll explain the basics of troubleshooting your vehicles problem.

To read the CEL, you will need an OBDII Code Reader/Scanner.   If you are unfamiliar with what an OBDII Code Reader/Scanner is and how to use it, please read our article How to Troubleshoot a Check Engine Light

Once you have performed a scan and brought up the codes you need to be able to interpret them.   Some codes may be self explanatory, while other codes may only indicate that the engine is not operating correctly.   This is because, in order to keep a modern engine running as cleanly as possible, the Engine Control Unit (ECU), constantly monitors the oxygen content in the exhaust system and adjusts the fuel and air mixture as required.  It also monitors such diverse parameters as engine RPM, coolant temperature, inlet air temperature and many more to calculate as accurately as possible the correct fuel air mixture for the engine.  If a sensor fails outright, then the ECU will log a code saying that the sensor has failed.  However, sometimes the component may not suffer a complete failure.   To the ECU it is still functioning correctly.  An example of the first, (simple to diagnose) type of code would be P0340 which is a generic code for a failure in the camshaft position sensor circuit.  This is very straightforward code to interpret.  It means either that the camshaft position sensor has failed, or there is an electrical fault in the wiring of that circuit.   On the other hand a code P0152 can be much more difficult to diagnose.   It means that the O2 sensor on bank 1 is showing a high voltage.   While this could be an indication that the O2 sensor is failing, it more likely that the engine is running rich and the O2 sensor is simply recording that this rich-running condition.  An example would be a faulty coolant temperature sensor that is providing the wrong information to the ECU.  If the faulty data suggests that the engine is running at a cooler temperature than it actually is, the ECU will think that the engine hasn’t warmed up and will richen the air:fuel mixture which will increase both HC and CO emissions.  This causes the O2 sensor in the exhaust to give a high reading, hence the P0152 code.  Because the temperature sensor hasn’t failed completely and is still giving a reading to the ECU, the ECU has no way of knowing that this is the fault.  It does know that the engine is running rich.   As there could be many different causes of a rich running condition, troubleshooting this particular code is going to take both more brain power, and more time.

Whatever trouble code you are faced with, if you intend to repair the car yourself, then the next step is to determine the actual source of the problem.   Even a simple code such as  P0340 may still require some investigation to determine the problem as it could be either a problem with the cam sensor, or it could be a fault in the wiring in that circuit. You don’t want to buy a new cam sensor only to find  out that it doesn’t cure the problem.  In the case of a code like P0152 troubleshooting will be more time consuming since you will need to eliminate possible causes one by one.  Depending on its specifications, your OBDII scanner may be able to assist you in this process.  While a very basic code reader will generally only give you the code and allow you to reset the CEL, more upmarket scanners such as the  Actron CP9690 allow you to tap into a database of reported repairs categorized by frequency and effectiveness.  This will provide you with a list of repairs that have succeeded in fixing the problem, beginning with the most common ones allowing you to focus you efforts on the most likely problem first.

actron cp9690 obd2 emissions passing tool
actron cp9690 obd2 emissions passing tool

If your scanner/code reader does not have this feature, then you will need to try and troubleshoot the problem yourself.  A good place to start is to go online and check internet forums devoted to the make and model of your vehicle and to search for posts by people whose vehicles have similar symptoms to yours.   It’s not unusual for particular models of cars to be susceptible to certain mechanical faults and if your problem is a common fault, you are likely to find plenty of information to that effect. 

When performing this more extensive type of troubleshooting you may also find it useful to obtain a copy of the factory service manual.  These can often be found in a downloadable format online.  These manuals will often have troubleshooting guides for common problems that will take you step by step through a process of elimination.

Because the engine management and emission control systems on modern cars are electronically controlled, one essential tool in tracing the source of the problem is a multimeter/component tester.   This allows you to test various sensors and other components to see if they are operating within spec. This can save you a lot of money by allowing you to eliminate those components which are working correctly, rather than simply buying components, installing them and hoping for the best.

Once you have determined the source of the problem you will need to effect the repair.  Once  this is completed you will need to clear the codes from the ECU using your OBDII Scanner.   Take the car for a drive to ensure that the CEL doesn’t come back on.     

One last note.  Modern OBD2 systems record when a trouble code has been reset and if an OBDII scan is require as part of the smog test, the vehicle will fail if the reset of the trouble codes has occurred very recently so you cannot simply drive up to the testing station, quickly reset the codes and hope for the best.  Because of this, it is important to plan ahead and perform any repairs and trouble code resets well in advance of the smog test so that when you attend the test station, an OBDII scan will that the repair was permanent in nature.


With modern engine management systems there is no reason to get stressed out about taking your car to a smog test.  If your CEL is not illuminated, odds are your car will pass the test with flying colours.   Even if it is illuminated, with a bit of work and basic tools you should be able to diagnose the problem and effect the repair yourself.   

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