Chapter 15 Fuel and Emission Control Systems 315 Copyright by Goodheart-Willcox Co., Inc. Figure 15-14. OBD II systems produce a four digit code with a letter designator. The breakdown of this code is shown here. B Figure 15-15. Using a noid light. A—Disconnect the wiring harness from the injector without damaging the connector. B—Install the noid light in the harness. The light should flash with the engine running. Connector B A • Freeze frame. Takes a snapshot of sensor and actuator values when a problem occurs. • Troubleshooting. Provides help and instructions for diagnosing faults. Caution: Make sure the scan tool is properly connected to the vehicle. Some technicians have mistakenly connected scan tools to the wrong connector (the tachometer connector, for example), which can damage the scan tool. Interpreting Trouble Codes Most technicians check the ECM for stored diagnostic trouble codes before performing tests on specific compo- nents. When verifying trouble code numbers, start from the lowest number and work up. For example, if codes 13 and 45 are displayed, correct trouble code 13 first and then 45. If multiple codes are stored, you can often eliminate one or more codes by using this method. Quite often, correct- ing one code will also fix the other code(s). For example, if a leaking fuel injector is repaired it can fix the code for a catalytic converter problem. OBD I codes are two digit numbers, such as 32. These numbers correspond with a particular problem listed in the service manual. OBD II codes are alphanumeric with a four digit number and a letter designator. The letter appears first and indicates in which system the problem is located. The first number indicates whether the code is SAE or manufac- turer specific. The second number indicates the function of the system where the fault is located. The last two numbers indicate the specific fault. Figure 15-14 gives a breakdown of the OBD II diagnostic code. Clearing Trouble Codes To clear or remove the trouble codes from an OBD I computer system, you must usually disconnect the battery or the fuse to the computer for about 10 seconds. You must use a scan tool to clear codes on vehicles with OBD II systems. After clearing the codes, check to see if the MIL comes back on after engine operation. If it does, this indicates there is still a problem in the system. Further tests are needed. Noid Light Injector Circuit Testing A noid light is a special test light for checking electronic fuel injector feed circuits. Different noid lights are made to fit the wiring harnesses on different vehicles. Usually the make of vehicle on which the noid light can be used is printed on the tool. A stethoscope can be used to tell which injector is dead. A dead injector will not produce a clicking sound. To use a noid light: 1. Disconnect the wiring harness from the dead fuel injector, Figure 15-15. Most harness connectors are a positive-lock type to keep the wiring from vibrating loose. Make sure you release the connector properly to prevent part damage.