DC DMV performs three different types of vehicle emissions tests; which test your vehicle will need depends on how old it is.
Idle Testing (two speeds): For vehicles from 1968 through 1983
Tail pipe emissions are measured for hydrocarbons (HC), carbon monoxide (CO) and carbon dioxide (CO2).
The test is comprised of two phases:
- An idle test at 350-1200 RPM (Revolution Per Minute)
- A high RPM test at 2200-2500 RPM (Revolution Per Minute)
IM 240: For vehicles from 1984–1995
Stands for "Inspection/Maintenance", an "enhanced" emissions testing program with a tailpipe test that lasts 240 seconds. The test is done on a dynamometer to simulate actual driving conditions. The vehicle is put through a "driving trace" as it accelerates, decelerates and cruises at various speeds. The emissions are collected at the tailpipe and analyzed by a computer to computer the total amount of pollutants in grams per mile (gpm) that are being emitted. The test measures carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOX). The I/M 240 test also includes a check of the vehicle's evaporative emissions control system to make sure that the fuel system is not leaking fuel vapors into the atmosphere and a flow test of the vehicle's canister purge control valve.
On-Board Diagnostics (OBD): For vehicles from 1996 and newer
The On-Board Diagnostic (OBDII) system tracks the vehicle’s performance through the use of “Monitors.” A monitor is a specific type of test that the OBDII system performs on certain emission related components or subsystems of the vehicle.
During the OBDII inspection process, the emissions inspection analyzer is connected to the vehicles DLC (Data Link Connector) and asks the vehicle’s OBDII system to provide the status of all of its OBDII monitors. If there are too many monitors that indicate “Not Ready,” the analyzer will reject the vehicle from testing.