Background

Readiness monitors, or emissions monitors, are self-diagnostic tools used by cars to self-check emissions systems. Readiness monitors can help the vehicle’s computer determine it its emissions systems are damaged or malfunctioning. After the active portion of the OBD-II test, the analyzer will indicate if any readiness monitors are unset. An unset readiness monitor indicates a lack of information stored in the vehicle’s computer. This means it’s possible that there may be an undiagnosed trouble code for that monitor. There may be a potential emission related problem with the corresponding system that would cause the vehicle to fail the test were the monitor completed. Current regulations permit vehicles built between 1996 and 2000 to have no more than 2 unset readiness monitors. Vehicles 2001 and newer can only have 1 unset readiness monitor. If a vehicle exceeds the maximum number of readiness monitors for its model year, the result will be a rejected test.

This screen shows all the monitors equipped on the vehicle and their status. In this case, all monitors are completed. If a vehicle is not at full operating temperature, monitors may not be active. **The Heated Catalyst in particular has to be at 800ºF to fully function.**

Readiness monitors run periodically, so a vehicle may fail because it hasn’t been driven enough prior to conducting the inspection. Information may also be cleared if the battery is disconnected, or if the charging system fails to adequately charge the battery. Any drain on the electrical system, such as a short or a broken wire will exacerbate the problem. If the vehicle is not driven for long periods of time, the battery may lose charge and monitors will become unset. Monitors may also become unset if an OBD-II reader is used to delete information stored in the ECU. In all of these cases, the vehicle will need to be driven enough to complete a drive cycle, banking enough information to complete the required unset monitors. This is achieved by driving under regular conditions for 3 to 7 days prior to the retest to allow the readiness monitors to collect enough information to set properly, or by completing specific driving instructions from the manufacturer to set each individual monitor. The time required to complete a drive cycle can vary by vehicle. If at this time the readiness monitors still haven’t run to completion, it is possible the vehicle will need repair. Remember that vehicles newer than 2001 can have one unset monitor and still pass. Vehicles built between 1996 and 2000 may have no more than two unset monitors and still pass emissions inspection.