[California98Civic]

Part of the text from Help my car failed the Smog check inspection - Smog Check Star Certified Roseville Sacramento Area (lightly edited)

Emissions:

Concentration of combustion products in the vehicle's exhaust, most of which pollute the air, give important diagnostic clues to the vehicle's engine function and condition. The component gases which contribute the most to air pollution are hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). This is not N2O (NITROUS). Three of the five gases measured at the tailpipe are regulated pollutants - HC, CO and NOx. The remaining gases, oxygen (O2) And carbon dioxide (CO2), while non-regulated, play a significant role as diagnostic aids. The four gas analyzer measures HC, CO, CO2 and O2 concentrations. The five gas analyzer (BAR 97) adds the measurement of NOx.

The exhaust gas analyzer is a highly accurate test instrument. It can assist in diagnosing, fuel, exhaust, emission control, and engine service problems. Following are a few general facts to keep in mind when using the gas analyzer:

1) High Carbon Monoxide (CO) readings usually indicate a fuel mixture richer than ideal (rich mixture - air fuel ratio below 14.7). In general CO is an indicator of combustion efficiency. The amount of CO in a vehicle’s exhaust is directly related to its air-fuel ratio. High CO levels result from inadequate O2 supply needed for complete combustion. This is caused by a too rich mixture - too much fuel (AFR readings below the optimal 14.7, Lambda below 1.0). If your vehicle failed the Smog Inspection and has a high C.O. level, This should always be fixed first. Circumstances that can lead to high CO emissions:

* Dirty or restricted air filters

*Excessively dirty or contaminated oil

*Saturated charcoal canister

*Non-Functioning PCV valve system

*Improper operation of the fuel delivery system

*Improperly functioning thermactor system

*Catalytic converter intervention and CO concentrations
High CO readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an CO reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. Low range C.O. readings are possible, and not uncommon, from a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter CO levels will be masked by the catalytic converter and the potential for a CO problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

2) Normal CO readings. If the combustion process is succeeding at or near the stoichiometric point (AFR equals 14.7, Lambda equals 1.0), C.O. levels during an idle test will typically measure less than 1% Pre-Catalytic Converter.

3) Low CO readings. There is, effectively, no reading for CO that can be characterized as too low or "below optimal". CO concentrations will appear "normal" even in a lean burning environment, where AFR is above 14.7 (Lambda is above 1.0).

4) High hydrocarbon (HC) readings usually indicate excessive unburned fuel caused by a lack of ignition or by incomplete combustion. ALWAYS FIX CO BEFORE HC. Concentrations are measured in parts per million (PPM). Common causes include a faulty ignition system, vacuum leaks, and fuel mixture problems. Circumstances that can lead to a high HC emissions are:

* Incomplete combustion due to fouled spark plugs.

* Improper timing or dwell

* Damaged ignition wires

* Low compression

* Vacuum leak

* Ineffective or faulty air management system (ECM control of air/fuel ratios)

* Catalytic converter intervention and HC concentrations
High HC readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an HC reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. HC readings at or near "normal" are possible, and not uncommon. From a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter HC levels will be masked by the catalytic converter and the potential for an HC problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

5) Oxygen (O2) readings. Oxygen, measured as a percentage of the exhaust volume, reflects the amount of gas remaining in the exhaust sample after the combustion process has taken place. Ambient O2 readings should be about 20%, reflecting the natural amount oxygen found in the air. The ideal range for vehicles without a secondary air injection system is less than 1.5%. If there is an air injection system, O2 levels will typically fall in the range of 3% to 4%. Pinching off the air hose of a vehicle equipped with air injection should produce O2 levels similar to those found for vehicles without air injection.

6) High oxygen (O2) readings indicate too lean an air-fuel ratio (AFR higher than 14.7, Lambda greater than 1.0). Circumstances that can lead to high O2 emissions are:

* Lean fuel mixture (AFR above 14.7)

* Vacuum leaks

* Ignition related problems causing misfires.

7) Low O2 indicates a rich fuel mixture (AFR below 14.7, Lambda below 1.0).

8) High carbon dioxide (CO2) readings indicate a nearly ideal air-fuel ratio and efficient combustion

9) Low carbon dioxide (CO2) readings indicate a fuel mixture either too rich or too lean, exhaust system leaks, or sample dilution.

10) Oxides of Nitrogen readings. Oxides of nitrogen (NOx), including nitric oxide (NO) and nitrous oxide (NO2), are formed if the combustion temperatures within the combustion chamber exceed some 2,500 degrees F. This can occur when the engine is under load. When excessive temperature conditions exist, the greatest amount of NOx is typically produced at the stoichiometric point (AFR 14.7 or Lambda of 1.0) as the engine is under a light load. If the combustion process within an engine is burning fuel at or near stoichiometric point, NOx levels on acceleration will typically read significantly higher than those measured at cruise and during deceleration. Typically, the NOx readings at idle will be 0 PPM.

11) High NOx Readings. Circumstances that can lead to abnormally high NOx emissions are:

* Malfunctioning EGR valve

* Lean fuel mixture (AFR above 14.7, Lambda above 1.0)

* Improper spark advance

* Thermostatic air heater stuck in the heated air position

* Missing or damaged cold air duct

* Combustion chamber deposits

* Malfunctioning catalytic converter

* Catalytic converter intervention and NOx concentrations
High NOx readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an NOx reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. NOx readings at or near "normal" are possible, and not uncommon. From a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter NOx levels will be masked by the catalytic converter and the potential for an NOx problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

12) Low NOx readings. There is, effectively, no reading for NOx that can be characterized as too low or below optimal. NOx is naturally 0 ppm at idle. NOx concentrations may appear normal even in a rich burning environment where the AFR is well below 14.7 (Lambda below 1.0).