Air Quality Division: Vehicle Emissions: Technical Information

Test Types

The model year and engine type of your vehicle determines which type of test your vehicle will receive. Below are brief descriptions of the tests:

  • IM 147: This is the enhanced test used for most 1981 through 1995 gasoline-powered vehicles in the metro Phoenix inspection program. In this test, the vehicle is driven on rollers (called a dynamometer) at varying speeds to simulate urban driving. The exhaust is continuously measured and the results are shown in grams per mile.

  • Steady State Loaded/Idle: This test is used for most 1967 through 1995 vehicles in Tucson and 1967 through 1980 models in Phoenix. The vehicle is tested at idle and (depending on vehicle class) at approximately 25 to 30 miles per hour. The results are shown in percent or parts per million.

  • On Board Diagnostics: Used on 1996 and newer light duty vehicles only, engine operating data is accessed by connecting directly to a computer in the vehicle that continuously monitors engine emission control systems operation. The on-board diagnostic test can identify problems before they lead to engine damage and emissions system failure.

  • Diesel: Diesel vehicles are tested for opacity (smoke density). The test uses an opacity meter, which is an instrument that measures the percentage of opacity of the exhaust. Light duty diesel vehicles and all Tucson area diesel vehicles are tested under load on a dynamometer. In metro Phoenix, heavy duty diesel vehicles are tested using a procedure called "snap acceleration" or "snap idle."

  • Tampering: In addition to one of the above tests, a vehicle may undergo a visual inspection to determine the presence of certain emission control components as installed by the manufacturer. Inspection includes catalytic converter, operational air pump, positive crankcase ventilation system and evaporative control system. These inspections coupled with the evaporative system integrity (pressure) test constitute the tampering inspection.

Fuel Additives and Retrofit Devices

A retrofit device or fuel additive is a product that, when installed on or added to the motor fuel of a vehicle, will decrease exhaust emissions and/or increase fuel economy. Inspection and maintenance (IM) data, literature, tapes and California Air Resources Board Exemptions have historically not supported the claims that a device reduces exhaust emissions or saves fuel.

ADEQ does not maintain a program to evaluate or certify the effectiveness of retrofit devices or fuel additives to reduce exhaust emissions. All inquiries for evaluation or certification are referred to U.S.EPA or the State of California; both of which maintain programs that provide technical assistance in the development of test protocol and the evaluation of the product effects on emissions/fuel economy. These programs use the complete federal test procedure, as described in the Code of Federal Regulations (CFR) , Title 40, Part 86, which is the only valid test for device evaluation.

Modification to a vehicle without using certified parts and following an approved process may result in I/M test failure. Installing a retrofit device on a vehicle without a reasonable basis for knowing that the device will not adversely affect a vehicle's emissions may result in civil penalties.

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