A vehicle may receive an alternative test for one of several reasons:
There are up to 11 readiness monitors in a vehicle that allow the vehicle’s Onboard Diagnostic system (computer system) to monitor for potential emissions problems. Some of these monitors run whenever the vehicle is operating while others run periodically. Once a monitor has run, it is set to “Ready” indicating that the system’s self-evaluation is complete.
Part of Colorado’s OBD emissions test requires certain monitors to be set to “Ready” to ensure the system has properly performed this self-evaluation. All OBD-inspected vehicles must have the Catalytic Converter, Oxygen Sensor, and Heated Oxygen Sensor (if so equipped) monitors set to “Ready” during the inspection. Vehicles model year 2000 and older can have two remaining monitors “Not Ready” while vehicles 2001 and newer can only have one unset monitor.
Specific conditions must be met while driving for the Readiness Monitors to be set to “Ready.” Some vehicle manufacturers provide specific driving procedures to set the necessary monitors to ready.
If you received a “Vehicle Unable to Test” form during your inspection, it was for any of the reasons checked by the inspector. Unsafe, inaccessible, or malfunctioning equipment must be repaired before the vehicle can be inspected.