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DIAGNOSE – EMISSIONS TEST FAILURE (Smog Check failure)
by Lance Wright


SYMPTOM SUMMARY

The vehicle has failed an emissions test or the exhaust emissions are too high as measured by an emissions analyzer.

USUAL CAUSEsmogtest
There are numerous components that if not working properly, can cause high emissions or an emissions test failure. You should read each of the components description and operation in the emission control system section of the manufacturer’s manual covering your vehicle in order to gain a working knowledge of each system. These systems are designed to reduce the emissions (as set by California emissions standards). If any of these emissions components fail, emissions can increase. Along with these systems, other components such as spark plugs, plug wires, and filters can also increase emissions if defective. Emissions or smog is a generic term used to describe certain harmful gases in the exhaust. Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOX) are usually measured in an emissions test. The state and federal government place certain limits on each of these harmful gases emitted. Carbon Dioxide (CO2) is also measured, but is not considered a harmful gas. When the measured gases from the vehicle exceed the state or federal limits, the vehicle fails the emissions test.

DIAGNOSIS
Diagnosis of high emissions can be an involved process that usually requires special tools and measuring equipment. However, armed with a test report indicating what gases are high can help you determine what systems may not be functional. If the vehicle has failed as a result of high CO and HC, the CO failure should be diagnosed first. If the “Service Engine Soon” or “Check Engine” light is on it should be diagnosed prior to any emissions diagnosis. CO Failure is the result of an excessively rich air fuel mixture. The following is a list of the most common causes:
– Defective Oxygen Sensor
– Inoperative Air Injection System (if equipped)
– Leaking or defective Fuel Injectors
– Restricted air filter (especially if high CO is present only at high RPM)
– Vacuum leaks resulting in improper MAP sensor operation
– Defective air mass or air flow sensor
– Malfunctioning fuel evaporation system or purge valve
– Defective Catalytic Converter
– Defective thermostat (cooling system), thermostat stuck open
– Oil contaminated with fuel, excessive miles between oil changes

HC Failure is the result of incomplete combustion. Any item that causes incomplete combustion can result in high HC. The most common causes are:
– Worn spark plugs
– Defective spark plug wires
– Worn distributor cap and/or rotor
– Improper ignition timing, usually over advanced
– Vacuum leaks
– Engine mechanical failure, low compression, worn valves, excessive oil consumption

NOX Failure is the result of combustion temperatures that are too high. The most common causes are:
– Inoperative Exhaust Gas Recirculation System (EGR)
– Cooling system malfunction, engine running too hot (restricted radiator, defective thermostat, etc.)
– Over advanced ignition timing
– Excessively lean air fuel mixture (defective oxygen sensor, MAP sensor or Air Mass Sensor)

PRECAUTIONS, TIPS, and NOTES
If the engine is due for a tune up it is wise to perform preventative maintenance prior to spending too much time diagnosing the emissions problem. Proper vehicle maintenance can go a long way in keeping a vehicle’s emissions within specifications. A complete tune-up should consist of: spark plugs, ditributor cap and rotor (if equipped), spark plug wires, air filter, fuel filter, and oil change. Although performing this service is no gaurantee of a passed emissions test, it certainly increases the likelyhood of success. If the vehicle has over 60,000 miles on it and has failed for the reason of high CO, installation of a new oxygen sensor might be a prudent investment especially if the vehicle is only equipped with one sensor.


(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

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12 Responses to “Diagnose Emission Test Failure”

  1. Cedric mathis says:

    I have a mitsubishi montero sport 2001 eengine 3.0 the asm emission test HChc((ppm) rpms 1742 max 45 ave. 4 measure 53@15mph 25mph rpm 1745 max28 ave 4measure36.

  2. Cedric mathis says:

    What is the cause failure

  3. Peter says:

    I have failed the ASm Emission Test recently to my 1999 Mercury Villager. The NO (PPM) MAX is 501, but the result of measure was 972 in 15 mph test and 25 mph test was 1003 to MAX 754. Why and how to take of it?
    Look forward to receiving your advice. Thanks.

  4. ashley says:

    My 04 frontier just failed a smog check. It shows the oxygen sensor passed but I failed the CO % and NO (ppm) categories. Should i go ahead and replace the oxygen sensors? Dont believe they have ever been changed since ive owned it. Thanks!!!

  5. DAVID BOYKIN says:

    i have a 1996 volkwagen cabro 2.0 engine type aba gasoline engine no egr.emission control system application are. pcv.evap.oxygen sensor.(LAMBDA) SYSTEM.THREE WAY CATALYYIC CONVERTER MY (NOx) is very high 1100. p.p,m can you help me please.retirer ase tech.

  6. craig says:

    the code po440 showed up when i had my chev s10 truck checked for emission . can u tell how to fix it . and get it to pass inspection ..

  7. JP says:

    Hi i have been desperately trying to fix my 94 LT1 corvette for the past few yrs. It misfires/knocks and looses power at high engine temps only, runs perfect when the car is cold. For some reason it is running extremely rich. My failed smog results are: CO2 13.9, O2 0.7, HC 244 @15mph & 122 @ 25mph, CO 0.47 & 0.51, NO 1061 & 716. Any help is much appreciated as to why, I just bought news cats but havent got a chance to install them. Thank you!

  8. [...] fails Emissions for CO http://www.auto-repair-help.com/auto…st_failure.php Some may or may not be irrelevant, but seem to be good info Hows the air injection pump or is it [...]

  9. Antonio montano says:

    I have 2003 chev silverado and fail emission test kore fail and mil command fail , what I can do , I don’t have many to send to fix ,I work jas part time .help.

  10. [...] site thatĀ specializesĀ in explaining the way a car runs created the article “DIAGNOSE – EMISSIONS TEST FAILURE (Smog Check failure)” to help explain why a car can fail an emissions test and require emission [...]

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