In other words, the computer has sensed an error from an emissions component, and replaced the data from the component with data from it's memory.
How is the Check Engine Light tested? During the emissions inspection the check engine light is tested two ways. The first is conducted by the smog technician, and the second by the smog machine. The smog machine test only applies to vehicles 1996 model and newer. We will explain why later.
Part 1. OBD I & II "Check Engine Light" Test: During the smog technician's functional test, he/she will be looking for a constant or intermediate illuminated check engine light, malfunction indicator lamp, or service engine soon light. By the way... all three of these lights are similar in terms of being engine emissions trouble lights. Your vehicle is equipped with only one of these types of lights. Ford Motor Company will typically choose to use a service engine soon light instead of a check engine light. Honda chooses to use the check engine light. Each manufacturer has a preference. They all do the same thing.
The smog technician or mechanic will be looking for an illuminated check engine light during the smog inspection. Any time the check engine light is illuminated while the engine is running causes an automatic smog test failure. The only time the technician wants to see the check engine light on is when the ignition is in the ON position, and engine not running.
If the check engine light is not illuminated while the ignition is in ON position and engine off, this too causes an immediate smog check failure. The fact that the check engine light does not turn on during ignition ON may be due to a defective emissions control computer and/or a defective light bulb (Check Engine Light Lamp - 12v).
Both are failures.
During the last phase of the smog test, the technician will be asked to enter your vehicle's check engine light visual results. He/She will enter the data as noted. Check engine light OFF or ON. That alone will determine your vehicle's success in passing the test or not.
Part 2. OBDII "Check Engine" Test: The second part of Check Engine Light test applies to 1996 or newer cars, trucks, SUVs, vans, and RVs only. These vehicles are equipped with an On Board Diagnostics (OBD II) system and input/output data link connecter (DLC). During the smog test your vehicle will be attached to the smog machine via it's OBD II DLC link connecter. The OBD II link will relay all "Check Engine" conditions along with stored trouble codes within it's database to the smog machine while the vehicle is being tested. If any trouble codes are present which caused the check engine light to illuminate either regularly or intermittently, the data will be sent to the smog machine via the data link connecter cable and the vehicle will fail the smog inspection.
The OBD II diagnostic system is designed to monitor all aspects of an engine's emissions control system, and report this information to a central database within the ECU (computer). This information is processed and checked against the computers pre-determined values for various inputs levels and performance patterns. If any problems are found, the computer will determine whether to alert the driver or not. If a decision has been made to alert the driver of an emissions problem, the "Check Engine" or "Engine Malfunction" light will illuminate on the vehicle's dashboard. In more serious emission conditions the computer may even begin to rapidly flash the "Check Engine/Malfunction" light indicating to the driver, that the vehicle needs immediate diagnosis/repair attention.
Part 3. OBDII "Readiness Flags" Test: Your 1996 and newer car, truck, van, SUV, or motorhome will not pass the smog test if certain "readiness flags" are not set. Some "check engine" related failures don't illuminate the check engine light, but do cause smog check failures. These faults are referred to as "readiness flag" faults. Readiness flags indicate that certain emissions systems which the OBD II computer has been monitoring have passed internal self monitoring tests, indicating that those systems are working properly. If the smog machine detects that there are certain readiness flags which have not set, the data will be relayed to the smog machine and your vehicle will fail the smog test.
In order to set all the proper Readiness Flags the OBDII system must complete at least one good drive cycle (in some cases two or three). A good drive cycle is a sequence of passing internal tests which the OBDII computer runs while your vehicle is being driven. This ensures all emissions systems are functioning properly. A drive cycle usually requires one to two weeks of ordinary everyday driving.
Readiness flag failures are often seen on vehicles which have had recent repairs requiring disconnecting of the battery, and/or the emissions computer. Disconnection of power to the ECU resets all readiness flags. These vehicles will need to be driven in order to reset the required flags.
Part 4. OBDII "Trouble Code" Test: This OBDII test is not applicable to 1995 and older vehicles. Trouble codes indicate that the OBD II computer has detected a problem within the emissions system. The trouble code will specifically indicate the component and problem which was found. Newer vehicle's have very complex codes in the thousands. Smog check repair centers can retrieve the trouble codes from the OBDII and inspect the component/s which the codes indicate.
OBD-II Trouble Code Lookup - How to extract trouble codes using a trouble code scanner, and what do the codes mean?
What if I Fail the Check Engine Light Test? There are three ways your car may fail the check engine light test. You may fail in one or all three of the categories below.
Failure A. Check engine light, malfunction indicator lamp or service engine soon light illuminated, and smog technician enters this information into the smog machine.
- Repair for Failure A. Using a trouble code scanner, the smog technician must extract the trouble codes causing the check engine light to illuminate, and diagnose the responsible emissions components hands-on. Once problem/s are repaired, the vehicle must be driven to set proper readiness flags, and can then be retested.
Failure B. Smog machine detects an active OBD-II trouble code during the smog test via the OBD-II data link connecter. Check engine light is not currently illuminated, but may have been in the past.
- Repair for Failure B. After the smog test is complete the smog technician will hand over a Vehicle Inspection Report or VIR. The VIR will have printed at the bottom the trouble code/s which were extracted by the smog machine. The smog technician must use these code/s the conduct a hands-on diagnosis of the components in question. Usually a diagnose fee of two labor hours will be required. Average labor rate is $65.00 per hour.
Note: The trouble code/s will only direct the smog technician to the general area of the system failure. It is up to an emissions repair mechanic to inspect the system and emissions components for proper operation, and repair faults as necessary.
Failure C. Smog machine detects certain required "readiness flags" are not set. The reason for the flags not being set depends on the select readiness flag. Two possibilities apply for all readiness flag test failures:
- Recent disconnection of battery (12v) system. This is common on vehicles which have recently required removal or disconnections of the vehicle's battery or engine control computer.
- Vehicle has an emissions system failure not allowing "readiness flag" self tests to run.
- Repair for Failure C. If readiness flags are not set because your car's battery was recently disconnected, and your car's malfunction indicator lamp, check engine light, or service engine soon light is not illuminated, the repair might be simple. Drive your vehicle for one week under normal driving conditions. During this period the emissions computer will run the proper readiness self tests to set the required "readiness flags". Your vehicle should not pass the smog check.
If this does not resolve the issue, or if the check engine light is illuminated, a more severe emissions failure may exist, requiring hands-on diagnosis by a smog check certified smog repair center.
How to erase trouble codes and turn off the check engine light
If you're performing this procedure to pass an emissions test when the check engine light, malfunction indicator lamp or service engine soon light is on or illuminated, read on and in detail to learn how the check engine light reset procedure works. The check engine light reset procedure applies to all vehicles.
You may have read information about how to turn off the check engine light or service engine soon light prior to the emissions test in order to pass the smog inspection, or emissions test. By turning off the check engine, malfunction indicator lamp or service engine soon light, in actuality deleting all emissions data from your vehicle's emissions computer, the smog machine will not know your vehicle has or has had emissions faults.
We will describe the reset or "turn off the light" procedure so you may understand the OBD (on board diagnostics) emissions testing process and how the check engine light or service engine soon light apply to the emissions test.
Warning! Perform this procedure at your own risk!
Step A. Locate your vehicle's battery. Disconnect the 12v negative cable (black wire) from battery terminal (-). Leave it disconnect for 10 minutes. While the battery is disconnected, cycle the ignition switch to ON position 3 times.
- Note you've just erased important emissions related data which is invaluable to a smog technician attempting to repair your vehicle.
- You also deleted very important "readiness flag" data which took many driving hours to compile.
Step B. Ensure ignition switch is OFF. Reconnect the battery 12v negative cable (black wire) to battery terminal (-).
Step C. Cycle ignition switch to ON position and wait 1 minute, then start your vehicle. Check Engine Light or Service Engine Soon Light should be reset and turned OFF.
- If you are successful in turning off the check engine light, this means your vehicle has not stored it's emissions data in the ROM section of it's computer and you have just deleted all data pertaining to your vehicle's engine and emissions systems, probably including your radio & system settings.
Step D. Drive your vehicle for one week under normal driving conditions. During this period the emissions computer is gathering data and re-learning your vehicle's emissions components and systems. The emissions computer OBDII (on-board diagnostics) system must complete at least one drive cycle (in some cases two or three). A drive cycle is a sequence of internal tests which the emissions computer runs while your vehicle is being driven. This ensures all emissions systems are functioning properly. Proper "readiness flags" are set as the computer completes it's cycles. Test cycles are unique to a vehicle. Certain cycles run under very strict parameters, and may require extended driving time to trigger on. Cycle data and readiness flag information is available through your dealership's service department. The data vary widely.
- During your drive, maybe even as soon as you start the engine, your vehicle realizes an emissions fault, and turns on the Check Engine or Service Engine Soon light.
- Emissions computer refuses to set a required readiness flag. No check engine light, but no chance of passing the emissions test either. However, you will not know this until your vehicle is smog tested. If the particular emissions test center doesn't offer a free re-test, you forfeit your inspection fee.
Step E. Take the smog test. Important Note! Your vehicle might not be completely ready for the smog check inspection. Required parameters may have not been set by the emissions computer.
- Your vehicle might not be ready for the emissions test. There will be no way of knowing this until the smog test is complete. Your normal driving pattern (if not for an extended length of time) may have failed to trigger the emissions computer tests needed to set the required readiness flags.
Will I pass the smog check after erasing my car's trouble codes?
What are my chances of passing the emissions test after erasing the emissions computer data and turning off the check engine light?
A slight chance of passing the emissions test after a reset exists, but it is very slim. The trick is to get your vehicle smog checked before your engine computer detects the check engine light illuminating trouble code, and only after the required readiness flags have set. The chances of the engine computer setting the required readiness flags before detecting the trouble code and illuminating the check engine light are slim to none. In our opinion, time is better spent finding a reputable smog check repair station to diagnose the check engine or service engine soon light, then time spent trying to avoid detection and/or repairs.
Even after evading check engine trouble detection, passing the emissions test will require your vehicle's exhaust sample, which will be collected by the smog machine, fall within California Air Resource Board specifications, and all required emissions components be functioning properly. Remember passing the entire smog test requires your vehicle pass the visual, functional, and emissions portions of the test. One may be able to hide trouble code information by deleting data, but the vehicle's exhaust will not be able to hide high emissions.
Important Note: California law allows only a vehicle's registered owner or a State certified smog repair station conduct emissions related repairs. We recommend you ensure the auto repair shop you visit is smog check certified. Smog check investigators will not get involved with faulty emissions repairs performed at non-emissions certified auto repair shops. Nor will they reimburse your costs if you end up applying for the CAP consumer assistance program.
C. Gas Cap & Filler Neck
During the smog test your vehicles gas cap and filler neck will also be inspected. They will have to be
of proper fit and design. The gas cap must be able to hold
pressure at factory specifications and the filler neck must
not be altered to accept leaded gas. This test is to ensure
the vehicle is not polluting fuel tank fumes through the
filler neck or using the wrong type of fuel. Most vehicles
pass this portion of the smog test.
The test is conducted by the smog machine. You will notice the smog technician remove your car's gas cap and attach it to a gas cap receptacle on the smog machine. The smog machine will then pressurize the system and record if pressure is lost through the gas cap. A small amount of loss is allowed. If your
vehicle's gas cap appears to be broken, does not fit properly, or has a broken or missing seal, it may fail the test.
What upsets consumers is that failing the gas cap test alone will cause the vehicle to fail the entire emissions test inspection. Even after all exhaust emissions pass successfully and no visual faults are found your vehicle will not pass the smog check.
The good side is the State of California will allow a vehicle owner to purchase a new gas cap from the smog station during the smog test. This allows the smog technician to continue the smog test, without having to fail the vehicle. The vehicle owner also has the choice of purchasing a new gas cap at a future date, however this would require the smog technician to fail the vehicle at the time of inspection. After a new cap is purchased a new smog inspection must be performed.
NOTE: A vehicle may fail the gas cap test, yet see no Check Engine light failure. The parameters required to pass the gas cap test and the parameters required to trigger the check engine light are not the same.
Loose Gas Cap Turning On the Check Engine Light - Late model vehicles have failed the emissions test due to a loose gas cap. Note however that a loose gas cap will trigger on the check engine light and you should not have your car inspected with the check engine light illuminated. The simple fault of not properly tightening the gas cap after filling up at a gas station has caused vehicles to fail the smog test.
This system works as such... the on board emissions computer runs a series of tests checking all major emissions systems and their components. The EVAP system is on the list. The computer runs a test on the EVAP system for vacuum. Note this is a very similar test to the LPFET test (which is discussed in section E of this page) which will be administered by the smog station, except it is done by your vehicle's emissions computer. A loose gas cap will not allow the EVAP system to hold proper vacuum indicating either broken EVAP vacuum lines and/or disconnected connections, or a loose gas cap.
D. Exhaust Gas Recirculation Valve (EGR)
During the smog test your vehicle's EGR valve will be inspected for proper operation. This test applies to vehicles which are administered the "basic" California emissions test. your vehicle will not be driven on a dynamometer, and it's EGR valve will be manually checked.
How The EGR Valve is Tested - The EGR valve test process and what to do if your car fails or failed the EGR valve check.
EGR stands for exhaust gas recirculation. The EGR system recirculates
exhaust gas back into the combustion chambers. Since
these recycled exhaust gases have already been in the combustion
chambers once, they have burned up most of their fuels, means there
is now much less real fuel in the chambers to ignite. This keeps the
chamber temperatures down and thus reduces NOx emissions. The EGR
valve should be inspected to ensure its proper operation. A working
valve should be able to open its passage using manifold vacuum.
Manifold vacuum is created during the engine's intake cycle. The
high demand for air during this cycle creates a vacuum within the
engine's intake manifold. This vacuum is then used to control
several important functions within the vehicle, including
controlling the EGR valve. Some vehicles even rely on this vacuum to
control their heating and air-conditioning components. The EGR
system is prone to collecting carbon build-up. Some vehicle
manufacturers recommend cleaning this component an a regular basis.
Please Click on "Under Your Hood" for more information on EGR valves
and testing procedures.
The following vehicles are
equipped with EGR systems - Acura, Audi, Buick, Chevrolet, Chrysler,
Dodge, Plymouth, Fiat, Ford, GM, GMC, Saturn, Honda, Hyundai,
Infiniti, Isuzu, Jaguar, Jeep, Lexus, Mazda, Mercedes Benz,
Mitsubishi, Nissan, Peugeot, Porsche, Renault, Land Range Rover,
Saab, Saturn, Subaru, Suzuki, Toyota, Volkswagen, VW, Volvo, and
E. Low Fuel Pressure Evaporative Test (LPFET)
The low pressure fuel evaporative test is a test of your vehicle's EVAP system. The LPFET test is administered in addition to the entire smog inspection. 1976
to 1995 model year vehicles; car, truck, van, SUV, RV and motorhome will be tested, which includes all pre OBDII vehicles subject to a smog check.
How is the LPFET test conducted? The LPFET test is conducted by a BAR approved Low Pressure Fuel Evaporative Test machine. A pressure line is attached to your car's gas filler neck. Your vehicle's EVAP system and gas tank will be pressurized 0.5-1.0 psi (14”-28” column of water) and measured for 2 minutes. The allowable drop is .40 psi. A greater then .40 psi drop indicates a leaking EVAP system. The LPFET machine will display a failure. The data will be entered into the smog machine, causing your vehicle to fail the emissions inspection.
This test is designed to
ensure your vehicle's fuel evaporative system is not leaking gas
fumes into the atmosphere. It is estimated that over 7,000,000
vehicles will need to be tested each year and of those 11% will
fail. The average cost to repair a failed EVAP system is estimated to be approximately $250.00. The LPFET will result in a reduction of emission and will improve air
Which vehicles are exempt from LPFET?
- Vehicles not originally equipped, and not required by state or federal law to be equipped, with a fuel evaporative control system.
- Vehicles with two or more fully operational fuel tanks or gas tanks.
- Vehicles with fuel evaporative lines not accessible without requiring the vehicle to be partially dismantled to gain access.
- Vehicles powered exclusively by compressed natural gas (CNG), liquefied petroleum gas (LPG), or liquid natural gas (LNG).