What We Test

repair For 2012, the AirCheck Davis County Program inspects all 1968 and newer light-, medium-, and heavy-duty vehicles licensed to operate on public roads. The inspections administered via the program identify gross polluting vehicles that emit excessive levels of harmful emissions. The AirCheck Davis County program mitigates the prevalence of these emissions as well as secondary pollutants such as ozone (O3) and very small inhalable particulates, and their impact on human health and our environment.

Depending on the vehicle model year, weight rating, and fuel type, the emissions analyzer selects the appropriate test combination for the vehicle undergoing emissions testing. To determine the exact type of emissions test that applies to a specific vehicle, contact Worldwide Environmental Products (WEP) at (801) 546-8860. Be prepared to provide the following information:
  • Vehicle Model Year
  • Vehicle Manufacturer
  • Vehicle Model
  • Vehicle Fuel Type
  • Gross Vehicle Weight Rating (GVWR)
  • Engine Size

The different vehicle emissions inspections that are used in the AirCheck Davis Program include:

Visual Tamper Inspection

The emissions inspector conducts a visual inspection of the emissions related systems and related components for unauthorized or illegal modifications. This visual inspection for tampering helps to identify vehicles which may be emitting excessive pollutants, but are not being properly evaluated because of an improper vehicle modification. The Vehicle Inspection Report (VIR) includes a Visual/Gas Cap results table that lists results of the visible smoke/emissions inspection, the gas cap pressure test and results of the tamper inspection.

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Visible Emissions Inspection

According to the Utah Administrative Code, rule R307-201-3 for visible emissions standards; No owner or operator of a gasoline powered engine or vehicle shall allow, cause or permit visible emissions and emissions from on-road diesel engines manufactured after January 1, 1973, shall be of a shade or density no darker than 20% opacity, except for starting motion no farther than 100 yards. During a vehicle emissions test, the inspector is required to verify visible emissions do not exceed allowable limits.

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Fuel Cap Inspection

Fuel tank filler cap testing is done to help ensure hydrocarbon (HC) vapors do not escape from the fuel storage tank to the atmosphere. Fuel cap testing checks the cap to filler neck seal, and any pressure relief valves that are part of the cap. Most caps incorporate positive and negative pressure relief valves that are designed to maintain an internal fuel storage tank pressure near atmospheric pressure. If the internal tank pressure gets too high due to fuel expansion and vaporization (caused by high heat conditions), the fuel tank may expand beyond the limits of the tank seals and cause a rupture. If the internal tank pressure gets too low (a vacuum created in the fuel tank) due to fuel cooling (caused by cool conditions), the fuel tank may collapse. To avoid these malfunctions of the fuel tank, the pressure and vacuum relief valves open at preset pressure conditions and maintain near atmospheric pressure conditions inside the tank regardless of fuel level or temperature conditions.

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Two Speed Idle (TSI) Test

TSI testing is done with the engine and exhaust system components (specifically the catalyst) fully warmed up and measures tailpipe hydrocarbon (HC) and carbon monoxide (CO) emissions in units of concentration, relative to the full exhaust stream. The exhaust sample probe is inserted into the tailpipe far enough to obtain an accurate sample. HCs are measure in parts per million (displayed on the VIR as PPM) and CO is measured in terms of percent (displayed on the VIR as %). The PPM and % standards vary, depending on vehicle model year and gross vehicle weight rating (GVWR). Two sets of values for HC and CO are obtained during the TSI test. One set of measurements are obtained during normal engine idle mode (curb idle, or an idle speed below 1200 engine RPM) and the other set of measurements are obtained during a high idle speed (approximately 2500 engine RPM).
The VIR displays 4 rows of information relating to the tailpipe emissions readings and includes:
  • Standard
  • Reading
  • Deviation
  • Result

  • Standard: Values listed in the Standard row represent the actual tailpipe emissions limits allowed for
    the specific vehicle. Emissions readings are not allowed to exceed the Standard limits.

  • Reading: Values listed for the Reading identify the actual measured emissions levels.

  • Deviation: Deviation values are the calculated difference between the Standard limit and the measured Reading values. Deviation amounts are determined by taking the Standard limit and subtracting the
    Reading value.

  • Result: Information in the Result row indicates whether or not the specific exhaust emission is lower
    than the Standard (Pass) or higher than the Standard (Fail). A Fail in any one or more of the four
    categories will result in the vehicle failing the emissions test.

  • CO2 (carbon dioxide) readings are included on the VIR. CO2 emissions are a result of complete
    combustion and readings between 13% - 15% indicate efficient combustion.

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OBD I/M Test

On-Board Diagnostics (OBD) emissions tests use information from the vehicle’s on-board emissions control computer system to determine if the vehicle passes or doesn’t pass the emissions test. The vehicle’s OBD system actually does the testing of the emissions control systems and components and passes the results to the emissions analyzer, which evaluates the data and based on federal and local regulations, determines if the vehicle passes or not.

For all vehicle being OBD tested, the Diagnostic Link Connector (DLC) must be accessible and operative, communication between the vehicle and the emissions test equipment must be possible, and the Malfunction Indicator Lamp (MIL) cannot be commanded ON. When the MIL is commanded ON, an emissions related malfunction is currently present and the vehicle will fail the emissions test.

Also, for vehicles being OBD tested the following standards exist for Readiness Codes (Monitors):
Vehicles model years 2001 and newer must have no more than 1 Readiness Monitor not ready, and,
Vehicles model years 1996 – 2000 are not allowed to have more than 2 Readiness Monitors not ready.

If the vehicle failed an initial test due to a catalyst related malfunction and the vehicle is being retested, the Catalyst Readiness Monitor must be ready.

Readiness Monitor, MIL, and Fault Code information is retrieved from the vehicle’s OBD system. KOEO and KOER results are manually entered by the emissions inspector and are based on the visual inspections of the MIL during those two operating conditions.

Fault Codes are typically referred to as Diagnostic Trouble Codes (DTCs) and are stored by the OBD system when an emissions malfunction is identified and confirmed as an actual malfunction. A Permanent Fault Code (Permanent DTC) can only be cleared from computer memory by the OBD system after the malfunction has been repaired and the OBD system has had the chance to test the related system and verified the repair has been effective

For more information on OBD systems, refer to OBDII Explained http://www.aircheckdavis.com/obd2Explained.aspx

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MIL Inspection

Visual Malfunction Indicator Lamp (MIL) checks inspect the MIL on the dashboard of the car to see if it is illuminated during the Key On Engine Off (KOEO) and Key On Engine Running (KOER) conditions. The MIL should illuminate as a bulb check during the KOEO operating condition. If no emissions-related malfunctions are currently present, the MIL will not be illuminated during the KOER operating condition. The results of these two visual inspections will be indicated on the Vehicle Inspection Report (VIR).

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Diesel Dynamometer Opacity Test

Diesel dynamometer opacity inspections are performed by securing the vehicle on a dynamometer and operating the vehicle under typical road load conditions. Opacity measurements are made at the end of a 10-second cruise of 50mph and also at the end of a 6-second acceleration phase. Opacity measurements take into account the amount of black smoke emitted from the exhaust and is a direct indication of very small particulates that pose significant health and environmental concerns. Opacity measurements are displayed on the vehicle inspection report (VIR) in a percent value. 0 percent indicates no measureable black smoke and 100 percent indicates no light is able to pass through the dense cloud of exhaust particulates. Opacity standards are set at a 20 percent limit, which are based on state and federal standards.

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Diesel Snap Acceleration Opacity Test

Diesel Snap Acceleration Opacity (Opacity) inspections are performed by first verifying the engine speed governor is functioning properly, and then throttling the engine to its maximum governed RPMs multiple times. Readings from the three (3) accelerations, or ""snaps"", are listed and compared to the applicable opacity limit. Vehicles 1995 model year and older are not allowed opacity amounts greater than 40% and vehicles 1996 and newer are limited to 20%.

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