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Emissions Testing: Will Your Car Pass?

Copyright AA1Car.com

Emissions testing or Smog testing is used in many areas of the country to improve air quality. But even if your community is not affected, you still need to know about emissions testing and smog checks because problems that often cause emission failures can also cause a variety of driveability and performance complaints.

Mandatory emissions testing has become a fact of life in many areas as a means of identifying vehicles that are "gross polluters" so they can be fixed. See States That Require Vehicle Emissions Testing to see if your state requires emissions testing. Such programs force vehicle owners to have emission problems repaired that might otherwise be ignored. So when a vehicle fails an emissions test or a smog check, most motorists are not exactly overjoyed to learn that repairs are needed.

Everybody is for clean air as long as somebody else pays for it. That is why all new cars and light trucks built from 1981 through 1995 have had a 5 year, 50,000 mile federal emissions warranty. This warranty covered all emission control components as well as the fuel delivery system (except the pump and filter), ignition system (except spark plugs), and engine management system including all sensors.

In 1995, the federal emissions warranty requirements changed. The warranty was extended to 8 years and 80,000 miles on the powertrain control module and catalytic converter, but rolled back to only 2 years and 24,000 miles on everything else. Even so, all of these components are still covered for 3 years or 36,000 miles (or longer) by the individual vehicle manufacturers bumper-to-bumper warranties. In California, the coverage is much longer.

Once a vehicle is out of warranty, the burden of paying for emission repairs becomes the sole responsibility of the vehicle owner.

Most inspection programs include "waiver" provisions that limit the amount of money motorists have to spend on emission repairs. If an emissions problem cannot be resolved within the specified waiver limit (which may be anywhere from $50 up to $450 depending on the local regulations and applicable model year), the vehicle gets a "pass" even though it may still be a polluter.

The objective, therefore, is to get the most bang for your repair buck (the most pollution reduction for the least out-of-pocket repair expense).

When a vehicle has multiple problems (one or more fouled or worn spark plugs, one or more bad plug wires, plus worn rings and valve guides), zero in on the repairs that will make the most noticeable improvement. A single misfiring plug, for example, can increase hydrocarbon emissions enormously (10 times normal!). Replacing the spark plugs (and plug wires if necessary) will eliminate the misfire problem (at least temporarily) and make a dramatic difference in reducing the engines overall emissions. The vehicle may still not be in compliance because the engine is burning oil, but it will run cleaner than it did before and have cost far less than an engine overhaul.


Each state or municipality determines its own cut points for emissions testing as well as the specific tests that must be performed. Most test programs look at only two pollutants: unburned hydrocarbons (HC) and carbon monoxide (CO). Most also measure carbon dioxide (CO2) but only for diagnostic purposes since CO2 is not a pollutant (though it is a "greenhouse gas" that may contribute to global warming).

Although most emission test programs have now gone to a simple OBD II plug-in test (which does NOT measure tailpipe emissions but checks your vehicle computer for any faults that might increase emissions), some inspection programs may require visual checks of various emissions-related equipment for evidence of tampering. These include:


The cut points for acceptable HC and CO levels are generally based on the emission standards that a vehicle was required to meet when it was new, so older vehicles have more lenient emission standards than new ones (see chart).


Model year...Typical Cut Points...Well-tuned engine

..............CO%.....HC ppm..........CO%.....HC ppm

pre-1968..... 7.5-12.5... 750-2000..... 2.0-3.0... 250-500

1969-70..... 7.0-11.0... 650-1250..... 1.5-2.5... 200-300

1971-74..... 5.0-9.0... 425-1200..... 1.0-1.5... 100-200

1975-79..... 3.0-6.5... 300-650..... 0.5-1.0... 50-100

1980........ 1.5-3.5... 275-600..... 0.3-1.0... 50-100

1981-93..... 1.0-2.5... 200-300..... 0.0-0.5... 10-50

1994 & up..... 1.0-1.5... 50-100..... 0.0-0.2... 02-20

Notice in the emissions cut point chart that the actual emissions produced by the average well-tuned engine are substantially less than the cut points required to pass an emissions test. The actual cut points are more lenient because the goal of emissions testing is to identify the gross polluters so they can be fixed and brought back into compliance.


When a vehicle fails an emissions test, the motorist usually receives a printout that show the test results of the vehicles emissions as well as the applicable cut points. From this, you can determine if too much HC and/or CO caused the vehicle to fail.

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings.

Carbon monoxide failures indicate an overly rich fuel mixture. On older carbureted engines without electronic feedback controls, look for things like a stuck choke, misadjusted or fuel saturated float or a rich idle mixture adjustment. On newer vehicles with electronic carburetors or fuel injection, the system may not be going into closed loop because of a bad coolant or oxygen sensor.

If both HC and CO are high, the vehicle may have a bad catalytic converter or an air pump problem. For more info on catalytic converters, Click Here for Troubleshooting a P0420 Catalyst Code

NOX failures are usually EGR-related, since the EGR system is primarily responsible for reducing oxides of nitrogen. But NOX emissions can also be caused by a bad three-way converter or a computer control system that remains in open loop.


There are five things you should always check on every vehicle that has a computerized engine control system:

1. Scan for fault codes. Any codes that are found need to be dealt with before anything else.

2. Make sure the system is going into closed loop. No change in loop status often indicates a coolant sensor problem.

3. Confirm that the system is alternating the fuel mixture between rich and lean. This is absolutely essential for the converter to function efficiently. You can do this by observing the O2 sensors output with a scan tool, or directly with a digital storage oscilloscope or voltmeter. If everything is okay, the sensor should be producing an oscillating voltage that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2500 rpm).

4. Confirm that the system responds normally to changes in the air/fuel mixture. To test the system response, pull off a vacuum hose to create an air leak (not too large or the engine will die). You should see an immediate voltage drop in the O2 sensor output, and a corresponding increase in injector dwell or mixture control dwell from the computer. Making the fuel mixture artificially rich by injecting some propane into the intake manifold should cause the O2 sensor output to rise and the computer to lean out the fuel mixture.

5. Use your scan tool to confirm all monitors have completed and that the vehicle is ready for emissions testing BEFORE it goes to a test station. If all monitors are ready, and no faults have been found, the vehicle should pass an OBD II plug-in emissions test.


Though a good technician can often diagnose and repair emission problems without having to actually check tailpipe emissions, it is becoming increasingly necessary today to have an infrared exhaust analyzer with at least three gas and preferably four gas (or even five gas) capability. Why? To baseline vehicle emissions for diagnostic purposes, and to verify that the repairs made eliminated or reduced the emissions problem.

Reading HC and CO at the tailpipe to diagnose emission problems may not give you the complete picture because the catalytic converter "masks" many problems by significantly lowering HC and CO in the exhaust. That is where a three- or four-gas analyzer comes in handy. The relative proportions of carbon dioxide and oxyten in the exhaust can reveal whether the air/fuel ratio is correct or not as well as other problems that affect engine performance and emissions.

As combustion efficiency decreases, the oxygen content in the exhaust rises and carbon dioxide falls. An engine that is running at a nearly ideal air/fuel ratio of 14.5:1 will show about 14.5 percent carbon dioxide and 2.5 percent oxygen in the exhaust. Carbon dioxide readings of less than about 13 percent and oxygen readings greater than about 4 or 5 percent indicate poor combustion efficiency. This translates to an over-rich or over-lean air/fuel ratio, poor compression, or an ignition problem.


Most vehicles that are in good running condition and properly maintained should pass an emissions test. In some cases, though, minor problems may cause the vehicle to fail an emissions test. These include:


In most areas that do emissions testing today, a simple plug-in OBD II test is used to check emissions on 1996 and newer vehicles. The plug-in test checks to see that the vehicle's OBDII system is functioning normally, that all the requires self-checks have been completed, and that they are NO fault codes in memory.

A vehicle will be rejected for testing if all of the required OBD II readiness monitors have not run. This may require driving the vehicle for several days until all the monitors have run.

The vehicle will also fail the test if (1) the test computer cannot establish communication with the vehicle PCM (defective or disabled diagnostic connector), (2) if the Malfunction Indicator Lamp (MIL) is on or inoperable, or if there are fault codes in the PCM.

If the OBD II system is working properly, the MIL is not on and there are no codes, the vehicle should pass theplug-in test.

emissions testing More Emissions Related Articles:

Understanding OBD II Driveability & Emissions Problems

All About Onboard Diagnostics II (OBD II)

OBD Monitor Not Ready

Advanced Diagnostics: Mode 06

Controller Area Network (CAN) Diagnostics

Basic Emission Control Systems Overview

Exhaust Emissions Diagnosis

Troubleshooting a P0420 Catalyst Code

Catalytic Converters

Positive Crankcase Ventilation (PCV)

Exhaust Gas Recirculation (EGR)

EVAP Evaporative Emission Control System

Making Sense of Engine Sensors

Oxygen (O2) Sensors

Wide Ratio Air Fuel (WRAF) Sensors

Sensing Emission Problems (O2 Sensors)

Emissions testing update

OBD II Emissions Testing

Evolution of I/M 240

Scan Tool Help


Most Common Trouble Codes (and what causes them)

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