658 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. At one time or another, every technician encounters a vehicle problem that seems impossible to fi x. He or she might replace a part that seems bad, only to fi nd the same annoying symptoms when the attempted repair is fi nished. Th e new part must then be removed and further diagnosis completed until the real “culprit” or problem source is found. Th is is when advanced diagnostic techniques come in handy. Th is chapter summarizes these advanced techniques, such as using scan tool datastream values, an oscilloscope, and an exhaust gas analyzer. You will learn how to read scope waveforms and how to evaluate exhaust gas analyzer readings for troubleshooting diffi cult-to-locate problems. As you will learn, advanced scan tools can display scope waveforms and can also connect to a module for monitoring the engine exhaust gas content. See Figure 34-1. By learning how to use these tools and a few “tricks of the trade,” even the most diffi cult problems can be located and corrected with minimum frustration. Strategy-Based Diagnostics Strategy-based diagnostics involves using a consistent, logical procedure to narrow down possible problem sources. Basically when performing strategy-based diagnostics, you verify the complaint, make preliminary checks, read service bulletins, perform service-manual recommended checks, repair the problem, and recheck system operation. See Figure 34-2. If the scan tool shows a diagnostic trouble code (DTC), you should test everything related to that circuit. Perform pinpoint tests to determine whether the wiring or an elec- tronic part is at fault. If no trouble codes are present, refer to the service manual. It will give specifi c instructions and specifi cations for fi nding the soft , or intermittent, failure. When diagnosing problems, evaluate the trouble codes and the vehicle’s symptoms. Use your knowledge of system Figure 34-2. Chart shows the basic steps for strategy-based diagnostics. Read through the chart carefully. (General Motors) Verify customer complaint Make preliminary checks Perform service manual recommended checks Check for TSBs Stored DTC(s) Symptom, no DTC(s) Follow published DTC diagnostics Follow published symptom diagnostics Analyze & develop diagnostics or call technical assistance No published diagnostics Intermittent See diagnostic details Operating as designed Customer misunderstanding of system: refer customer to management or zone Product problem: call technical assistance Isolate the root cause? No Reexamine the complaint Yes Repair & verify fix Figure 34-1. An advanced scan tool will not only display trouble codes and operating parameters, it will also show scope patterns for various components. A—This advanced scan tool comes with various cables, a CD, and program cartridges. It can show scope patterns and serve as an exhaust gas analyzer. B—Scan tool is showing a scope pattern for an idle air control motor and other live data. C—Scan tool is showing scope pattern for a mass airflow sensor and is giving information for troubleshooting this component. (OTC/SPX Corp.) A B C