Emissions Glossary

Airflow Sensor

Fuel-injected engines use an airflow sensor or meter to measure how much air is being drawn into the engine. The device measures the volume or mass of air being drawn into the engine and sends that data to the vehicle's on-board computer where it is used to calculate how much fuel is needed to maintain the desired air/fuel ratio. See also: Mass Air Flow Sensor, Vane Air Flow Sensor

Air-Fuel Mixture

This phrase refers to the mixture of air and atomized fuel that goes into the engine's cylinders for combustion. To burn efficiently, the mixture must be the correct ratio of air and fuel. See Air Fuel Ratio.

Air-Fuel Ratio

The mixture of air and fuel that goes into the engine's cylinders for combustion must contain a certain amount of fuel in order to burn efficiently. The volume of fuel relative to the volume of air in the mixture is called the air fuel ratio. If the ratio is just right (stoichiometric air-fuel ratio), the mixture burns the most efficiently and produces lower exhaust emissions. If there is too much fuel (rich air-fuel ratio), not all of the fuel will be burned completely and high carbon monoxide emissions will result. If there is not enough fuel (lean air-fuel ratio), the combustion of the mixture may stop prematurely and high hydrocarbon emissions will result.

Air-Fuel Sensor (See Oxygen Sensor)

Air Injection System

The air injection system injects air into the exhaust stream. The purpose of this is to promote further oxidation of the exhaust, namely the HC and CO emissions. A common misconception is that the injected air just dilutes the exhaust. In fact, it oxidizes HC and CO into H2O and CO2. Air injection systems include various types of components that affect where and when air is injected into the exhaust system. In many modern vehicles, the air injection system operates only during warm-up.

Air Temp Sensor

Many fuel-injected engines use an air temperature sensor (ATS) to measure the temperature of the air being drawn into the engine. The measured air temperature is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired air/fuel ratio.

Carbon Dioxide (CO2)

Carbon dioxide is a colorless, odorless gas produced by all animals, plants, fungi and microorganisms during respiration and is used by plants during photosynthesis. CO2 is a normal result of the combustion of fuels. CO2 is the main greenhouse gas produced by human activity.

Carbon Monoxide (CO)

Carbon monoxide (CO) is a poisonous gas that is formed when a carbon fuel is burned incompletely. In motor vehicle emissions, a high CO reading means too much fuel. Fuel can only come from three sources, the crankcase control system, the evaporative control system, or the actual fuel delivery system.

Carburetor

This is the main fuel delivery component used on most engines built up to the 1988 model year. Carburetors have several sub-systems that cause a different amount of fuel to be delivered to the engine under different operating conditions. A richer than normal mixture is needed during rapid acceleration and also for about the first two minutes of operation after a cold start.

Catalytic Converter

A pollution reduction device found on all modern vehicle exhaust systems that contains a catalyst. When heated to operating temperature, the catalytic converter converts the harmful gases (HC, CO and NOx) into into less harmful emissions including oxygen, carbon dioxide, and water.

Charcoal Canister

This is the main component of a vehicle's evaporative emission control system. This system controls the escape of fuel from the fuel tank. The charcoal canister stores the until it can be burned in the engine at an appropriate time. All vehicles built since the early seventies are equipped with a charcoal canister.

Check Engine Light

1996 and newer light-duty vehicles have On-Board Diagnostic (OBD) systems. If your vehicle finds a problem, it will illuminate the Malfunction Indicator Lamp or MIL, which is commonly called the ‘Check Engine Light’. Read more about OBD.

Combustion Chamber

An area inside the engine where air fuel mixture is drawn into, compressed, and then burned (combustion) to produce power to propel the vehicle. If the combustion chamber has accumulated carbon deposits, high NOx emissions may result.

Compression

The degree to which air/fuel mixture is compressed in an engine's cylinder prior to being ignited. Insufficient compression will prevent proper combustion and power output. A compression test measures each cylinder's compression pressure during cranking conditions.

Coolant Temp Sensor

All fuel-injected engines use a coolant temperature sensor (CTS) to measure the temperature of the engine. The measured temperature is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain smooth engine operation when the engine is cold and during warm-up.

Crankcase Ventilation Control System

When an engine is running, oil vapors (and possibly fuel vapors) exists in the crankcase. On all vehicles built since the late 1960s, the Crankcase Ventilation Control System prevents this from escaping into the atmosphere. See also PCV Valve.

Data Link Connector (DLC)

The interface between a vehicle's OBD computer and the OBD scanner. The DLC is usually located under the dashboard on the driver's side. Connecting an OBD scanner to the DLC allows inspectors and repair technicians to read the readiness status of the vehicle's various on-board monitors as well as any diagnostic trouble codes (DTCs).

Diagnostic Trouble Code (DTC)

Diagnostic trouble code (DTC) is an electronic signal stored in a vehicle's On-Board Diagnostic (OBD) computer, indicating the presence of a fault. 1996 and newer light-duty vehicles are equipped with an OBD system. This is a diagnostic system that continuously checks the condition and operation of key emissions control components and emissions-related systems in your vehicle.

If your vehicle finds an emissions-related problem, it will illuminate the Malfunction Indicator Lamp and store the DTC(s). All manufacturers use the same DTC naming standards. For example, a P0171 Code, means the same thing on a Ford as it does in a Hyundai or BMW.

As OBD systems have been around since 1996, most repair technicians have the equipment and training necessary to read diagnostic trouble codes.

EGR (See Exhaust Gas Recirculation)

Emissions Control Equipment

This is a phrase that generally refers to one or several components that are designed to reduce or prevent harmful emissions from a vehicle. Some examples of common emissions control equipment or systems are catalytic converters, oxygen sensors, EGR valves, fuel-injection systems, air injection systems, and gas caps.

Evaporative Control System

Fuel vapors are prevented from escaping into the atmosphere by the Evaporative Control System. The gas cap, tank, charcoal canister, and related hoses and valves are all part of the system.

Exhaust Gas Recirculation

Some engines are equipped with a device or system that recirculates a small amount of exhaust gas back through the combustion cycle under certain operating conditions. EGR systems cool the temperature of combustion, and result in reduced formation of NOx emissions.

Exhaust System

A system of components that directs burned gases from the engine, to the rear of the vehicle, and out into the atmosphere. Typical components of the exhaust system include the exhaust manifold (bolted to the engine), exhaust pipes (connecting the exhaust components to each other), and exhaust muffler (the device that reduces the engine exhaust noise). All vehicles built since 1988 (and most built since 1975) also have a catalytic converter in the exhaust. The catalytic converter is the only component of the exhaust system that reduces harmful emissions (the oxygen sensor is attached to the exhaust system but is not generally considered to be a part of the exhaust system).

Fast Idle Test

An exhaust emissions test conducted with the engine of the vehicle running under an accelerated condition to an extent (rpm) so prescribed.

Federal Emissions Warranty

Federal emissions warranty is two year/24,000 miles on most emissions components and eight years/80,000 miles on the catalytic converter and computer.

Fuel Cap (See Gas Cap)

Feedback Fuel System

A type of fuel system that is capable of monitoring the air fuel ratio and adjusting it to maintain maximum efficiency. Feedback fuel systems always have an oxygen sensor or an air/fuel sensor to monitor the engine exhaust and provide feedback to the vehicle's on-board computer. Every light-duty vehicle built since the 1988 model year (and some before that) is equipped with a feedback fuel system.

Fuel Delivery System

A general reference to the group of components on an engine that controls how much fuel goes into an engine for combustion. A vehicle's fuel delivery system could be a carburetor, a feedback carburetor, or a fuel injection system. In the case of alternative fuel systems such as natural gas or propane, the carburetor may be called a mixer.

Fuel Injection System

A system of vehicle components that deliver atomized fuel to the engine through small orifices that are usually opened electrically. Fuel injection systems have replaced carburetors on modern engines because they are much more accurate, consistent, and reliable.

Gas Cap

The removable cap that allows you to fill your vehicle's tank with fuel. The gas cap is an emission control device because it allows air to be drawn into the tank but prevents fuel s from escaping into the atmosphere.

Gas Cap Test

The gas cap test is a functional test of the gas cap's ability to prevent fuel vapors from escaping into the atmosphere. To pass the test, the cap must hold pressure and must not leak at a rate greater than 60 cubic centimeters per minute. Most vehicles up to and including 1997 model year receive a gas cap test as part of the ADEQ inspection. Vehicles newer than 1997 model year that have leaking gas caps are identified using an on-board computer test rather than the traditional gas cap test.

Hydrocarbons (HC)

Hydrocarbon (HC) exhaust emissions are a product of incomplete combustion of a hydrocarbon fuel such as gasoline, diesel, natural gas, and propane. Hydrocarbon emissions can also occur from evaporation of liquid hydrocarbons. HC emissions contribute to the formation of ground level ozone which can cause serious damage to human health and vegetation. See also Causes of Excess Hydrocarbons.

Curb Idle Test

An exhaust emissions test conducted with the engine of a vehicle running at the manufacturer's specified idle speed plus or minus one hundred revolutions per minute but without pressure exerted on the accelerator.

IDLE CO

Carbon monoxide emission measured at idle rpm.

IDLE HC

Hydrocarbon emission measured at idle rpm.

Ignition System

Gasoline, natural gas, and propane-fueled engines all require an electrical spark to ignite the air fuel mixture and produce power. The collection of components that generates the spark and controls when it occurs is referred to as the ignition system. Typical components included in an ignition system are spark plugs, ignition coils, and trigger devices such as crank position sensors. Ignition systems on older vehicles require regular maintenance. Modern ignition systems are extremely reliable and typically require less frequent attention than older vehicles.

IM240/IM147

The IM240 is a vehicle emissions test procedure developed by the US EPA. The test measures emission levels of HC, CO, and NOx while the vehicle is driven over a pre-defined series of accelerations, decelerations, and cruise conditions. The test may last up to 240 seconds depending on how "clean" the vehicle is in the early part of the test. The IM147 is a 147 second version (an abbreviated version) of the original 240 performed by the State of Arizona. Tests show that the IM147 takes less time to perform and produces more accurate results.

Induction System

The induction system is made up of hoses and ducting that may be rubber, plastic or metal, all of which direct outside air into the engine.

Intake Manifold

The part of the engine between the throttle plate and the cylinder head. The intake manifold directs air and fuel to each of the engines cylinders.

Injection Pump

The main component controlling the injection of fuel into each cylinder of a diesel-fueled engine.

Injection Timing

In a diesel-fueled engine, the time (in relation to piston position) when fuel is injected into the cylinder. If injection timing is incorrect, high opacity (smoke) emissions can occur.

Lean (Air-Fuel Mixture)

A condition where the air-fuel mixture is not the correct ratio (not enough fuel relative to the amount of air). High HC and/or high NOx emissions are symptoms of a lean air fuel ratio.

Loaded test

An exhaust emissions test conducted at cruise or transient conditions, as prescribed by code.

Malfunction Indicator Lamp (MIL)

An illuminated MIL is your vehicle's way of telling you it has found a problem related to the emissions control or engine management systems. The MIL in your car will look similar to one of the examples shown below.

Manufacturers can only use the MIL for indication of emission related faults, and not for maintenance reminders, mileage intervals, or any other non-emission-related items. In other words, if your MIL is illuminated, a fault has been detected that is considered by the vehicle manufacturer to be emissions related.

Generally, the OBD system is required to illuminate the MIL after the same fault has been found in two different driving cycles, which helps to avoid MIL illumination for random faults or abnormal conditions.

If your MIL is commanded on, take it to a repair technician to have the problem addressed before it becomes more serious.

Please note, 1996 and newer light-duty vehicles with a MIL commanded on by the OBD system (see MIL Command Status) will automatically fail an ADEQ inspection (does not apply to 1995 and older vehicles, heavy-duty vehicles, or alternate-fuelled vehicles).

MAP Sensor

The Manifold Air Pressure (MAP) Sensor closely measures and reports pressure changes withing the engine's induction system. The sensor converts pressure into voltage Depending on the design of the vehicle, the engine management computer may use this data to help adjust the air:fuel ratio; and/or it may use this data to verify proper function of such devices as the EGR, Purge Control Valve, Secondary Air Injection, etc.

Mass Air Flow Sensor

MIL Command Status

When the OBD system finds a problem related to the emissions control or engine management systems, it will illuminate the MIL and set a flag that indicates it is trying to illuminate the MIL. The purpose of the MIL Command Status indicator is to account for the possibility of a MIL bulb not working, or for it to be on due to an electrical fault. That is why the MIL Command Status indicator (rather than the MIL itself) is used to determine whether a vehicle has an emissions defect or not.

NOx (See Oxides of Nitrogen)

O2 Sensor (See Oxygen Sensor)

On-Board Computer

All vehicles built since 1988 (and some before that) are equipped with a computer that is designed to control the fuel delivery, ignition, and emission control systems on the engine. The most important function of the on-board computer is to continually adjust the air-fuel ratio so that the engine runs with maximum efficiency (lowest emissions, maximum fuel economy, maximum power, and smoothest idle). See also Feedback Fuel Control.

On-Board Diagnostic (OBD) System

All 1996 and newer light-duty vehicles are manufactured with an On-Board Diagnostic (OBD) system. All manufacturers use the same type of connector, communication protocol and trouble code-naming standard. The OBD system continuously checks the operation of key emissions control components and emissions-related systems in your vehicle.

If the system detects a fault that could cause emissions to exceed one and a half times the federal new vehicle standard for the model year and type, the dashboard Malfunction Indicator Lamp (MIL) will be illuminated on command from the Powertrain Control Module (PCM). Note that only emissions-related defects will cause the MIL to be commanded on. Other types of defects, that cannot affect emissions, will not cause the MIL to be commanded on.

OBD Test

The On-Board Diagnostic Test, or OBD test, is an emissions test that utilizes a vehicle's built-in On-Board Diagnostic (OBD) System which continuously checks the condition and operation of key emissions control components and emissions-related systems. All 1996 and newer light-duty vehicles come equipped with an OBD system.

If the vehicle's Malfunction Indicator Lamp (MIL) is commanded on, the test system will download and record the faults stored by the vehicle's OBD system. This information will be printed on your Vehicle Inspection Report and the vehicle will receive a fail result (does not apply to 1997 and older vehicles, heavy-duty vehicles, or alternate-fuelled vehicles).

If your vehicle's OBD system has completed all of its system checks (no more than one readiness monitor Not Ready) and the MIL is not commanded on, the vehicle will pass the OBD inspection.

For a more detailed description of the OBD test procedure, click here.

OEM (See Original Equipment Manufacturer)

Opacity

Opacity is a measurement, on a percentage scale, of how much light is blocked by the particles in the exhaust plume. Light smoke has a lower opacity than dark smoke. ADEQ inspection centers measure the opacity of diesel vehicles using theD147 test procedure. See also Causes of Excess Opacity.

Original Equipment Manufacturer

A general term referring to replacement auto parts made by the same company that manufactured the original part.

Oxidation Catalytic Converter

There are two types of catalytic converters in terms of what their function is: oxidation catalysts oxidize HC and CO. See also Three-Way Catalytic Converter.

Oxides of Nitrogen (NOx)

Oxides of Nitrogen (NOx) emissions occur when fuels are burned at high temperature. Some of the nitrogen (N2) in the air combines with some of the oxygen (O2) in the air to form nitric oxide (NO). In an engine, some of the NO undergoes additional reactions and turns into nitrogen dioxide (NO2). The emissions of NO and NO2 are collectively referred to as NOx. In addition to contributing to the formation of ozone, NOx emissions also lead to a buildup of nitrogen dioxide levels in the atmosphere, which are known to increase the risk of respiratory disease in children. See also Causes of Excess Oxides of Nitrogen.

Oxygen Sensor

A key component in feedback fuel systems is called the Oxygen (O2) Sensor. Located in the exhaust stream, the O2 Sensor detects the relative amount of oxygen that remains after combustion in an engine. The O2 sensor is electrically connected to the on-board computer, which interprets the signal and adjusts the air-fuel ratio as necessary to ensure maximum efficiency.

Positive Crankcase Ventilation (PCV)

On all vehicles built since the late 1960s, the Crankcase Ventilation Control System prevents crankcase s from escaping into the atmosphere. To do this, a regulated amount of vapor is introduced into the engine where it is burned. A key component in regulating this vapor is a PCV Valve or a PCV orifice.

Powertrain Control Module (PCM)

All 1996 and newer light-duty vehicles are manufactured with an On-Board Diagnostic (OBD) system that continuously monitors components that affect emission performance. The on-board computer responsible for this is called the Powertrain Control Module or PCM. The PCM is a computer that is designed to control the fuel delivery, ignition, and emission control systems on the engine. The most important function of the on-board computer is to continually adjust the air-fuel ratio so that the engine runs with maximum efficiency (lowest emissions, maximum fuel economy, maximum power, and smoothest idle).

Readiness Monitors (System Checks)

1996 and newer light-duty vehicles have on board computer monitoring systems. In addition to engine management, the onboard computer also verifies the proper function of emissions control systems by performing Emissions Control System Checks. System Checks are performed under certain conditions defined by the manufacturer and known as a “Drive Cycle”. The on board computer keeps track of whether the different emissions control System Checks have been completed. Upon completion of each individual System Check, the computer flags that specific emissions control system as “READY”. If the System Check has not yet been completed, the emissions control system flags that individual System Check as NOT READY. Readiness Monitors only confirm that the emissions control System Check has been performed; it does not test whether the system or devices within the system have passed or failed.

Readiness Monitor - "NOT READY" means the self diagnostic System Check has not completed.

Readiness Monitor - "READY" means the self diagnostic System Check has been completed.

NOTE: Regarding Vehicle Emissions Testing in Arizona, a vehicle will be rejected from emissions testing if: on 1996 - 2000 model year vehicles, more than two monitors report NOT READY; or, on 2001 or newer model year vehicles, more than one monitor reports NOT READY.

Rich (Air-Fuel Mixture)

A condition where the air-fuel mixture is not the correct ratio (too much fuel relative to the amount of air). High carbon monoxide emissions are a symptom of a rich air fuel mixture.

Speed Density

The term “Speed Density” refers to the method estimating the volume of air flowing into an engine is estimated using the engine's displacement, volumetric efficiency (VE) table values, engine RPM and manifold absolute pressure (MAP).

Stoichiometric (Air-Fuel Ratio)

When the ratio of air and fuel is precisely correct, it is referred to as stoichiometric. It is critical that air fuel ratio be stoichiometric in order for the mixture to be burned with maximum efficiency and for the catalytic converter to operate with maximum efficiency.

Tampering

Removal, disconnection, or alteration of any emissions control device which was installed at the time a vehicle was manufactured.

Three-Way Catalytic Converter

There are two types of catalytic converters in terms of what their function is: three-way catalysts (TWC) reduce NOx and oxidize HC and CO. See also Oxidation Catalytic Converter.

Throttle Position Sensor

A throttle is the part on an engine that opens up when the driver steps on the gas pedal. Many fuel-injected engines use a throttle position sensor to determine the driver's demand for power. The measured throttle position is indicated to the vehicle's on-board computer where it is used to calculate how much fuel should be delivered to maintain the desired air-fuel ratio.

Vacuum Leak

When an engine is running, a vacuum is created in the intake manifold between each of the cylinders in the engine and the throttle. Vacuum Leak refers to leakage in this area. Vacuum leaks can cause disruption of the air-fuel mixture on its way to the cylinders and can result in excessive HC emissions or CO emissions.

Vane Air Flow Sensor

2500rpm CO

Carbon monoxide emissions at measured at 2500 rpm with no load.

2500rpm HC

Hydrocarbon emissions at measured at 2500 rpm with no load.

Emissions Repair Phoenix Arizona - Apex Automotive