"INTRODUCTION This report of the Study of Mandatory Engine Maintenance for Reducing Vehicle Emissions summarizes results obtained during the first year of the Extended Phase One Emission Test Program. The main objective of this work was to assess the feasibility of reducing exhaust emissions through mandatory vehicle inspection and maintenance. The Extended Phase One effort involved the following activities: 1) Testing of 450 privately owned vehicles to measure both emission and engine adjustment deterioration rates over five four-month intervals. Of the five test periods@ two are complete. 2) Testing of 16 pre 1966 and post 1970 vehicles to establish specific relationships between engine settings or adjustments and exhaust emissions. (These relationships were developed for 1966-1970 vehicles during the previous program.) 3) Testing of 260 privately owned vehicles in the Detroit area to compare their frequency and extent of engine maladjustments with results previously obtained in the Los Angeles area. 4) Evaluation of the effectiveness with which service organizations maintain a selected class of engine malfunctions and maladjustments. 5) Upgrading the Economic Effectiveness Model and using it to evaluate proposed inspection/maintenance procedures. Items 2) through 5) have been completed. The remaining work is the completion of the engine deterioration tests@ the final updating of the Economic Effectiveness Model using the new test data@ and the selection of preferred inspection/maintenance procedures based on the completed test series and the updated model. The results obtained to date are reported in detail in Volumes II through VI. The material of these reports forms an essential background for any state or regional air pollution agency considering mandatory inspection/maintenance to meet the new Federal ambient air quality standards. The conclusions of the Extended Phase One Program tend to reinforce those given in the Interim Report in January 1972@ but are based upon a broader data base which includes: ? a more representative vehicle population including pre 1966 vehicles which are not emission controlled@ 1966-1970 emission controlled vehicles and 1971 NOx controlled vehicles (State of California) ? experimental data on repair effectiveness and maintenance deterioration rates ? all emission predictions based upon 1972 Federal Emission Test Procedures. The difficulty of achieving positive correction of faulty engine adjustments became apparent in the current phase as a result of studying repair effectiveness. The more promising results of the Interim Report were based on the assumption of ""perfect maintenance""@ which is clearly optimistic. Nonetheless@ there are definite indications that significant emission reductions can be achieved@ particularly in the pre 1971 vehicles@ through an emissions inspection followed by corrective maintenance. The best strategy is strongly dependent on the regional objectives and the consequent importance assigned to the reduction of each emission specie. Because of the nature of automotive emissions measurements and control (including legislation@ equipment design and inspection/maintenance criteria)@ it is strongly recommended that any proposed mandatory maintenance program be tested on a pilot basis before it is widely instituted."