Chapter 15 Fuel and Emission Control Systems 313 Copyright by Goodheart-Willcox Co., Inc. Throttle body problems Damaged fuel supply line Restricted fuel return line Clogged fuel filter Bad fuel pump Contaminated or leaking fuel tank Fuel injector problems Vacuum leaks Faulty fuel pressure regulator Bad cold start valve Improper adjustments Figure 15-12. Note the types of problems that can occur in a fuel injection system. in-system components, their related wiring, and the computer itself. The computer actually scans its input and output circuits to detect an incorrect voltage, resistance, or current. For example, with OBD II, engine misfires and fuel sys- tem malfunctions will cause the malfunction indicator light (MIL) in the dash to flash on and off. The misfire or poor mixture could damage the catalytic converter. The flashing MIL warns the driver that the vehicle could be damaged and should be serviced immediately. OBD II systems have four levels of diagnostic trouble codes (DTC): • Type A codes are emissions related. The MIL will illu- minate or flash when present. • Type B codes are also emissions related. However, the ECM will illuminate the MIL only when this type of code appears on two consecutive keystarts or “trips.” • Type C codes are non-emissions related. The ECM will not illuminate the MIL, however, it will store a DTC and illuminate a “service lamp” or the service message on vehicle equipped with a driver’s information center. • Type D codes are non-emissions related. The ECM will store a DTC, but will not illuminate any lamps. Data link connections, trouble codes, sensor and out- put device terminology, and scan tool capabilities are also standardized with OBD II. In the past, a service facility needed to have data link connectors for each make of vehicle serviced. Sometimes, multiple connectors were needed on a single vehicle. To solve this problem, the federal govern- ment and the Society of Automotive Engineers (SAE) has set standards for all vehicle manufacturers to use. Diagnostic Problem Indicators While each manufacturer’s systems have their differ- ences, they are similar in many ways. Diagnostic systems indicate problems as follows: • With OBD II systems, the ECM illuminates an MIL or check engine light on the dash. OBD II systems flash the MIL if a problem exists that could damage the vehicle’s emission system. When the MIL light illuminates, it tells the driver and technician that a problem exists. • With OBD I systems, the ECM displays a digital number code in the dash climate control or driver information center. • On some older import vehicles, the ECM illuminates LEDs on the computer itself to indicate problem codes.