Auto Repair Help

by Lance Wright


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

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 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)

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.)

62 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…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 […]

  11. kevin says:

    I have a 95 Volkswagen passage and its failing emissions because the hc ppm reading is 774 and 120 is allowed. Also my Co readings were high causing me to fail. Co was 1.29 and .67 allowed. Please help!

  12. c hall says:

    I have a 95 civic coupe with a 1.6L d16z6. I am failing Texas emissions test with a high HC level and high Nox level.

    Passing HC level at high speed is 147 and low speed is 152.
    First test high speed result 284, low speed 295.
    Second test high speed 279, low speed 306.

    Passing Nox level at high speed is 1072 and low speed is 1186.
    First test high speed result 5979, low speed 7098.
    Second test high speed 4927, low speed 5628

    Any advice or ideas would be greatly appreciated! Thanks!

  13. jayjayjay says:

    My honda accord 96 failed the emmisiin test and they hughligthed these. .Ac refrigerant /oxygen sensor/ catalytic converter eff. But. Yesterday I disconnect the battery because I install a new car stereo and subwoofer. Do u think disconnecting the battery effect these 3 highlighted problem. Please help.ty

  14. Very Informative post coming from a veteran auto shop owner of 30 years! I am sure this will help a lot of car and truck mechanics in diagnosing emission problems.

  15. mark harvey says:

    Failed emission test, test results (readiness fail,DTC3)could u please tell me what that means for a Toyota corolla 2003

  16. Mona says:

    Hey my name is mona I failed the emission test due to this code p0420, can u please tell me what that mean

  17. Mona says:

    Dodge Durango 1999,5.9L

  18. Jun Cabrera says:

    I have a 1992 BMW 325 IS that did not pass the HC, CO and NOX emissions test. What would be the estimated cost to have all these repaired. Thanks for any response.

  19. daniel says:

    i have a 92 dodge grand caravan 3.3L v6 it fail smog do to emissions test when tested at 15 mph max 976 and it came out 1077 what can i do to get it down to 976

  20. Heidi says:

    1999 Chevy Astro Van 2wd
    No ppm
    15 mph Max 501 MEAS 914
    25 mph Max 754 MEAS 910

  21. crystal says:

    Test code under gas is 1 …what is this?

  22. […] sensor might be a prudent investment especially if the vehicle is only equipped with one sensor.…st_failure.php __________________ OBD1/2 SOHC/DOHC VTEC HARNESS MINI ME SUBHARNESS 02 EXTENSION HARNESS […]

  23. ray gordon says:

    have 1991 dodge 150 5.2 eng. failed smog high rpm test.
    changed oil & filter r&r egr and pcv valves. cleaned spark plugs & air filter.
    high speed co 7.39
    high speed hc 223

  24. Eddie says:

    my no ppm reading 1106 allowed 945

  25. Charles M says:

    2002 Dodge Dakota emission failing due to cylinder compression issue. Shop says the cylinder compression is low and wont pass emissions, this is due to overheating. Is there a fix that will get me through emissions testing?

  26. Veggie says:

    I have a 2001 Mitsubishi eclipse GT. It failed SMOG due to OBDII monitors not ready. The smog tech asked me to drive another 100 miles and it should take care of by itself. Does it make sense ?

    • Mogeenk says:

      I have a 2002 Honda Accord special edition.It failed smog due to OBDII not ready.I drove on freeway 500 miles cannot pass. I drove 5 steps do it twices for sure.

      1)Begin idly engine 2.5 min,, Put on hand brakeand engage in D4 another 2.5 min with Air/con
      and rear de froster on.
      2)Closed A/C and the rear de froster Accelerate 60 miles 3)hold constant for3min.
      4)de celerated to 20 mph without brake,,stop
      5) accelerated to 60 mph hold constant for 5 min
      decelerated with out brake until stop.
      I did it 2 cycles and pass the test.
      I got those cycle from Honda internet and drive on 91 between van buren and university exit.
      The smog tech ask me 50$ more because he recommend to drive more and more ,no miracle. 06/06/2016

      • Mogeenk says:

        How to pass OBDII not ready.
        The Idea is to drive at constant speed 60mph 5mins.on freeway drive after big Truck is more practical on 91 freeway back and forth 2-4 times Ipass the test Because the engine have time to check their own system….Before that I drive to LA,CA at 80mph more than 500 miles.i fail the test every times.This is real life experiences,,,,,

        • Mogeenk says:

          Here is a typical Honda Accord drive cycle. Driving your Honda Accord in the manner below should make the required Emission Monitors “Ready”. Before you start the Drive Cycle make sure your Honda Accord’s fuel level is between 1/4 and 3/4, otherwise the EVAP test won’t run. Make sure to obey all traffic laws while performing the drive cycle.

          1. Begin by idling the engine while 2 1/2 minutes. Next idle for another 2 1/2 minutes with the A/C compressor and rear defroster on and while the transmission is in Drive (D); only of automatic transmission. This part of the drive cycle checks the oxygen sensor heater circuits, air pump and EVAP purge system.

          2. Turn the A/C compressor off and rear window defroster off and accelerate to 55 mph at half throttle. During this step the OBDII system checks for ignition misfire, fuel trim and EVAP canister purge.

          3. Hold at a steady state speed of 55 mph for 3 minutes. The computer will now be testing the EGR system, air injection pump, O2 sensors and canister purge.

          4. Decelerate to 20 mph without braking (or depressing the clutch). Once again the computer system will check EGR and purge functions.

          5. Accelerate back to 55 to 60 mph at half throttle. During this step the OBDII system will look for misfires, fuel trim and EVAP purge.

          6. Hold at a steady speed of 55 to 60 mph for 5 minutes. Catalytic converter efficiency, EGR flow, ignition misfire, fuel trim, EVAP system and oxygen sensors operation will now be checked.

          7. Decelerate to a stop without braking (or depressing clutch). The OBDII system will complete the drive cycle at this point.

          We recommend completing this drive cycle at least twice in order to insure all required parameters have been met.

          posted by SmogTips Support 08-02-2014

  27. john says:

    I have a 2006 chevy cobalt. Is there a sensor near the gas cap?

  28. Mason says:

    I have a 1991 gmc sierra sle my truck failed emissions. My truck failed Hydrocarbon you have to have under 1.60 my truck got a 2.06, i would like to know if you have any suggestions as to what i could do to make the hydrocarbon go back down. I was told to give it an oil change which has been done. Just any other ideas out there thanks…

  29. Jeikob says:

    1996 Honda Accord. High readings for co, hc, and nox. What is most likely the problem and how to fix?

  30. Matt says:

    I have a 1990 Nissan 240sx with a ka24e. I keep failing emissions because they can not get any reading out of the exhaust. Any ideas on what could cause this?

  31. Dawna says:

    I have a 1986 Subaru gl. I took it for an emissions test and it failed… CO GPM was 38.1896 and is not suppose to go above 20.000. What do I do. Please em me at banksdawna74 at gmail. Thanks. I’m desperate…

  32. Tina says:

    My evaul system has a tiny hole in it and i can not pass th emission tst How can i plug the hole up and what to do to find the hole

  33. Tim says:

    Before I tried to smog my 1999 Blazer, I changed the catalytic converter with a nonapproved one purchased on line. The smog check passed but the number on the cat didn’t start with a D. I changed cat with a approved one ,took it back. Now it won,t pass do to PO171.System too lean Bank 1. I changed the O2 sensors and the light still comes on. ???????”????

  34. maya how says:

    Hello, I can’t get my em1 Honda civic to pass the emissions CO2, its the high idle and low idle, there nearly 5 times over. This is my 7th time for the nut and my heart is broken with it, changed injectors , plugs and lots more, the lamba is perfect, just the CO2, any help you could give me . Thank you

  35. Mauro m. says:

    I hace a 2000 expedition I took it forma an emission test and it failed…Con gpm was 84.5093 andas is not suppose yo go above 15.0000 andas HC gpm was 2.0274 andas is not suppose yo go abov 1.2000 what do I do please en me ay regiomartinez@gmail.con Thanks

  36. J9 says:

    I have a 1995 Honda Accord I bought in Feb. Had major surgery so couldn’t drive, so I found out by DMV I couldn’t change title until smog check. Just got back and they said it failed NO (PPM) 15 mph 487 passed but the 25 mph max 516 my car measured 1420. They also said whoever sold it to me took it to be checked and it failed. I’m a single mom, just moved here last August and needed a car to get my son back and forth to school and sports. Real dumb, I didn’t know you cant sell without smog check.
    Just barely making it, and they said I need to take it to a mechanic. If possible, can you please let me know what I need to fix, so hopefully I can try myself. DMV sent me a notice and said I had to get it fixed or they will tow away. Cant walk well so this is really important to me.
    Hope you can help.
    Thank you………

  37. Matt says:

    …on what planet will a failed T-stat cause a NOx failure?

    We’re concerned with combustion FLAME-FRONT temperature. Do you really think a few degrees, relatively speaking, of engine temperature is going to have a large effect on this?

    Sickens me to see this myth perpetuated…

  38. It would be helpful to know when Lance wrote his interesting essay DIAGNOSE – Emissions Test Failure.
    Obviously it followed his retirement in 2006, but when did he write it?

  39. Tim says:

    I Have a 99′ chevy malibu V6, 3.1L- every year I pass the smog test, but this time it did not- I have spent some money on obd2 device’s, they do work but at a price. Also the code i’m throwing is 1441 emissions non-purge well this could be a lot of testing and finding, so far turns out the cat went bad not to say the ox- sensors bank 1 and 2 bad after checking. What next, keep checking looking for leaks just plain problems with all this junk on my car. OK it’s an old car but it’s been good to me and this smog crap is not going to change what’s next my obd2 says it not ready yet or a drive mode, don’t have that kind of time if it dose not meet the standards of smog WHAT A CROCK. This may be a trip to the DMV in 16 years to see what can be done..because soon will be to late, so let’s get this done..up for ideas?

  40. Jimmy says:

    I have a 1998 Oldsmobile Bravada and when I took the emissions test everything checked out except one thing was causing me to fail and that was the EGR valve monitor was failing to reset itself and what they told me to do was to just drive it some more well to make a long story short I completed about 10 drive cycles and put an extra 300 miles on my vehicle and it’s still not passing does anyone have any other advise other than just drive it more

  41. ExArtyM109A6 says:

    1994 Jeep Cherokee 4.0 failed CA smog due to Hi NO (1176) at 15 mph (Max=517) while within limits (355) NO at 25 mph (Max=539)

    Had new CAT installed, fuel filter, 02 sensors 2 years ago (less then 6,000 miles)

    Has had some idle issues in the recent past idling to high at start up if air conditioner is on at start. Every once in a while will feel like it’s starting to stall at stop light.

    No check engine light dealer said no stored codes when tested

    I’m leaning towards Fuel Pressure Regulator and/or Idle Air Control Valve

    1. Can Idle Air Control Valve cause smog test failure?

    2. What would you suggest me checking/replacing?

  42. Julie says:

    My 1993 2500 Chevy truck just failed CO only on emissions and only at low speed (passed at high speed). What could be the problem?

    Also tell truck passed HC and NOx. It only failed CO at the low speed.

  43. Brenda says:

    I have a ford ranger 1997 and it failed CO GPM limits is 15.00000 and I got 21.7975 what’s the problem

  44. Chris says:

    I have 1998 BMW M3 and my smog has failed my HC are 54 MAX , AVE 9, 152 MEA and NO @ 430 MAX AVE 67 572 MEA@15mph but @ 25mph everything on my 2nd smog inspection, I performed a tune up, ran fuel injection cleaning through a tank of gas and it got worse. Do you have suggestions on what I could do?

  45. Sam Solo says:

    I like the part where you talked about improper ignition timing as an issue with incomplete combustion. I could see how a lapse in the timing belt could cause an issue with proper fuel ignition. I’ll have to remember to make sure my timing belt is okay if I ever want to pass.

  46. Vickie says:

    I have a 1995 Mercury Villager and would like to know where the fuel filter is located?

  47. 2007 nisson armanda no engine light and everything pass but a code P.O. 463. level sensor fuel. i don’t understand why it didn’t pass??? themo fluid will do the trick?? true or not. Auto Zone employee said it works?? HELP! PLEASE. THANKS

  48. SCOTTY FRERE says:


  49. Mary says:

    My car everything is good on the emission I replace 2 sensors but I drove it 100 miles and it’s still not ready .

  50. terrence says:

    I have a 2016 Mercedes GLC250 , it fail the emission test:

    NOX 0.093 g/km (limit 0.06 g/km)
    PM 5.572 mg/km (limit 4.5 mg/km)

    anyone can advise and help me.

  51. I have a 98 Honda civic ex coupe failing emissions only on HC.
    New spark plugs,
    New spark plug wires,
    New distributor,
    Heavy-duty cap and rotor,
    New thermostat,
    New knock sensor,
    New Timing belt,
    New water pump,
    New O2 sensor,
    Cleaned injectors,
    New fuel pressure regulator,
    New oil,
    New oil filter,
    New air filter,
    1 year old cathalitic converter,
    Vacuum above 30
    No vacuum leaks.
    Compression above 170 all 4 cylinders,
    Engine doesn’t burn oil.
    And it runs smooth and strong.
    Technician test my car and while he was doing it he turn the AC on.
    After the test my car failed HC and also NO.
    I did ask why technician turn the AC on and they did give me the face like “shit he found out he was doing it”.
    So they tested my car again.
    This time only failed HC.
    Whit exactly the same amount 85 ppm on 15 mph and 25 mph.
    Last time my car passed the 15mph with half of the maximum allowed.
    So yesterday added a MAP sensor to the list of new parts and let’s see next week when I take it to smog again…

  52. Bob says:

    2005 Chevy Colbalt connected to the emissions test keeps coming up RPM’s Not registering make sure engine is running but the car is running & connected ???

  53. Jay Cee says:

    Will non working driver side locks and non working electrical windows cause inspection failure?

  54. Tammy Rice says:

    very well done! you have explain information regarding DIAGNOSIS very well.. thank you so much!

  55. frank spinden says:

    Aloha, my 1989 jeep Cherokee failed low speed co2, with a reading of 1.63, limit is 1.20, I replaced ecu temp sensor and oxy sensor, still reads high, say suggestions?

Leave a Reply

Your email address will not be published. Required fields are marked *