314 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. REVIEW CODES ** KEY ON, ENGINE OFF CODES-FIX FIRST ** 54 AIR CHARGE TEMP SIGNAL HIGH—CKT OPEN ** CONTINUOUS MEMORY CODES-FIX LAST ** A B Intake air temperature sensor Figure 15-13. Most technicians check for stored trouble codes using a scan tool. A—This scan tool readout shows a problem with the intake air temperature sensor. B—The technician would then check that sensor and its wiring for problems. Diagnostic Trouble Codes A diagnostic trouble code (DTC) is a digital signal produced and stored by the computer when an operat- ing parameter is exceeded. An operating parameter is an acceptable minimum and maximum value. The param- eter might be an acceptable voltage range from the oxygen sensor, a resistance range for a temperature sensor, current draw from a fuel injector coil winding, or an operational state from a monitored device. In any case, the computer stores the operating parameters for most inputs and outputs in permanent memory chips. Note: Most computer system problems are not related to the computer, such as a loose electrical connection or mechanical problem. Only about 20% of all performance problems are caused by an actual fault in the computer or one of its sensors. For this reason, always check for the most common problems before testing more complex computer- controlled components. Reading OBD I Codes If the output is an OBD I digital display, simply read the number and compare it with the chart in the service manual. If the output is a code flashed from the dashboard MIL or ECM mounted LED, count how many flashes occur between each pause. Write down the number and compare it to the service manual. If an analog meter must be used on an early model vehicle, count the number of needle deflec- tions between pauses. Write down the number and compare it to the chart in the service manual. Each system may vary. The service manual will give you instructions for the make and model of vehicle. Data Link Connector (Diagnostic Connector) The data link connector (DLC) is a multipin terminal used to link the scan tool to the computer. In the past, this connector was identified by a variety of names, including diagnostic connector and assembly line diagnostic link (ALDL). OBD I diagnostic connectors come in various shapes and sizes and have a varying number of pins or terminals. With OBD II, the DLC is a standardized, 16-pin connector. The female half of the connector is on the vehicle the male half is on the scan tool cable. The diagnostic con- nector may be located: • Under the left side of the dash, within arms reach when sitting in the driver’s seat. This is the standard OBD II location. • Near the firewall in the engine compartment, near or on the side of the fuse box, near the inner fender panel in the engine compartment, or under the center console on OBD I systems. Using Scan Tools A scan tool can be used to quickly check for prob- lems in the engine and its support systems, transmission, suspension system, anti-lock brake system, and other vehicle systems. This has greatly simplified the troubleshooting of complex automotive systems. OBD II codes can only be read with a scan tool. Modern scan tools provide prompts or step-by-step instructions in their display windows, Figure 15-13. The prompts tell you how to input specific vehicle information and run diagnostic tests. The scan tool may ask you to input VIN information. The VIN lets the scan tool know which engine, transmission, and options are installed on the vehicle. With some makes, however, the on-board com- puter contains this data and the scan tool will automatically download it. After the scan tool is set up, you can select which information you want displayed. Some of the infor- mation you can request includes: • Stored diagnostic trouble codes. The trouble code numbers of any stored diagnostic trouble codes are displayed. • Fault description. Explains what each stored diagnostic code means. This information is given with the trouble code number on most scan tools. • Datastream information. Displays the operating values of all monitored circuits and sensors. • Run tests. Performs sensor and actuator tests. • Oxygen sensor monitoring. Performs detailed tests of the O2 sensor signal. • Failure record. Lists the number of times a particular trouble code has occurred by keystarts or warmups.