When Twin City Auto performs a maintenance tune-up, we remove and replace the spark plugs, distributor cap and rotor (if non DIS), Positive crankcase ventilation valve, and fuel filter. Our technicians also clean the throttle plates on port fuel injected engines, and check or adjust the timing (non DIS). The O2 sensor(s) are then pin-point tested at the sensor in which the fuel mixture is artificially driven rich and lean while the O2(s) voltage is monitored for the correct response and response time. This procedure will keep the overall performance, dependability and fuel economy at its greatest. The recommended service intervals are: 24,000 miles-city, 36,000 miles-highway.