Introduction

2-01 Summarize the diagnostic process for engine performance issues.

Strategy-based diagnosis is a proven, systematic approach that has been used successfully to diagnose automotive system faults. Although experience may occasionally help with repeat or trending faults, this unfortunately does not hold true for many concerns. Therefore, a process is required to ensure the diagnosis is quick and efficient. The process involves nine steps, beginning with the most important step: verifying the customer concern. After confirming the concern, perform quick checks before carrying out in-depth diagnostic system testing. This process eliminates systems and components that are known to be good to find the root cause of the complaint. All original equipment manufacturers (OEMs) of vehicles recommend a diagnostic procedure, but they may vary slightly. This chapter combines real-world methods and a summary of those plans to establish a guideline for efficient diagnosis, to reduce the number of “good parts” replaced while searching for a cause.

Symptom-to-system-to-component-to-cause (SSCC) provides an organized, logical approach to diagnostics. In some instances, a fault is found before completing the entire procedure. Information gathered from the visual inspection or technical service bulletin (TSB) search may be all that is required. Other symptoms may require further, more involved testing that includes the entire SSCC/diagnostic process (FIGURE 2-1).

FIGURE 2-1 This diagnostic process is a simplistic way of looking at concerns with a customer’s vehicle.

Step 1: Verify the Customer Concern

Verifying the concern is the first, most important, and most overlooked step of the procedure. Failure to verify the concern leads to guessing and to unnecessary, costly parts replacement. Verification involves obtaining as much information as possible from the customer as to when and how the fault occurs. Collection of this information includes the service advisor gathering as much information as possible during the write-up process.

The following are some of the questions that a service advisor should ask the customer about their concern:

• When does the concern occur?
• How often does it occur?
• Under what operating conditions does it occur?
• How long does it last?
• Do external weather conditions—such as hot or cold temperatures, rain, snow, sitting overnight, or undergoing a hot soak—affect it?
• Have any aftermarket accessories been installed on the vehicle?
• Has the vehicle been purchased recently (this may indicate that the customer may not know the aftermarket accessories that have been installed on the vehicle)?

Some repair facilities provide a questionnaire that the customer fills out describing the condition and when it occurs (FIGURE 2-2).

Verifying the concern also assumes the technician assigned to the vehicle understands the “normal” operation of the system. Understanding system operation may require additional information gathered from service information, the owner’s manual, the customer, experience, or other additional sources. If the technician does not understand the operation of the system under inspection, their ability to repair the vehicle on the first attempt will be severely limited. It is next to impossible to “fix” something without an understanding of whether or not it is operating correctly.

FIGURE 2-2 Understanding the customer complaint facilitates diagnosis. Clear communication allows for a quick, direct diagnosis of the problem.

To replicate and verify the customer concern, testing may be done in the service bay or a road test may be required. Before performing a test drive, ensure the vehicle is safe to drive. Task another technician with monitoring a scan tool or other test equipment to prevent accidents. Distracted driving is against the law in most states, and doing so in a customer’s vehicle will compound the wrongdoing.

Step 2: Ensure the Vehicle Is Operating as Designed

At times, a customer concern may be normal; the vehicle is operating as it should, also called operating as designed (OAD). This step involves experience and understanding system operation. Making a comparison against a known-good vehicle, if available, can be done while operating under similar conditions, as described by the customer, to verify an actual fault. Consulting the owner’s manual or finding an owner’s manual for the vehicle will help educate the customer on proper vehicle operation. A customer who has just purchased a vehicle may not be familiar with the system’s operation and the rationale for its design (FIGURE 2-3). Vehicle content and operation change frequently, so even though they bought the “same vehicle,” which is only a couple of years newer, the system operation may have changed from what they are accustomed to. Customer-induced concerns are also a possibility. For example, refueling the vehicle with the engine running may set an evaporative emissions (EVAP) code.

FIGURE 2-3 Some complaints are due to the customer not understanding the capabilities of their vehicle.

Step 3: Diagnostic System Checks

This step verifies the proper operation of the system in question. At this point, install a scanner to gather diagnostic trouble codes (DTCs) from all modules. Gathering codes is an important diagnostic step due to the vehicle’s network. A module not tested may have a fault present that affects the operation of another module not believed to be a part of that system. After receiving DTCs, view and record freeze-frame or failure records before clearing the codes. Erasing DTCs also removes this valuable information (FIGURE 2-4).

FIGURE 2-4 Retrieving DTCs will direct the technician as they diagnose the concerns and faults with the vehicle.

When receiving multiple DTCs, follow the manufacturer’s recommended service procedures. Faults that result in a code can cause other DTCs to set, making the correct order of repair vital. Just retrieving a DTC does not lead to a proper repair. Understanding how to interpret the DTCs and the order of repair is equally important. Information about the DTCs set may also provide a diagnostic path—information such as correlation codes, system faults, non-component faults, and internal module failures are examples. Repairing and diagnosing DTCs in a random order may lead to extended diagnostic time and replacing parts that are not faulty. If information does not exist for repairing multiple codes, an industry rule is to begin with the first code listed on the scan tool. After correcting that code, clear the remaining codes, then retest to determine whether any codes repeat.

TECHNICIAN TIP

When diagnosing modern vehicles, always pull codes for engine performance concerns, including a no-start condition. Codes may be available for fuel, ignition, or antitheft system components; circuits; or operation.

Step 4: Preliminary Checks

Another of the steps that is often overlooked and that may lead to the root cause of the concern is the preliminary check. Perform a thorough visual inspection, looking for corroded connections, wires, blown fuses, aftermarket accessories, leaks, and missing or improperly installed parts. Do not disturb anything unless it is an obvious fault. Intermittent concerns can “mysteriously” disappear when harnesses, hoses, and gas caps are moved or tightened during the inspection process. Listen for unusual sounds, vibrations, and movements while operating the vehicle. Also be aware of any unusual odors, such as from coolant or oil burning, electrical burning, or musty/mildew odors (indicating possible water leaks). If possible, review the vehicle’s service history, either through questioning the customer or by searching for service records (TABLE 2-1).

TABLE 3-1 SDS Section Descriptions

•Check for obvious problems: corrosion, leaks, odors, vibrations, and excessive movement.

•Verify what does and does not work; a failure in one system may affect another.

•Inspect for signs of previous repairs and improper installation of parts.

•Check and verify proper battery voltage and charging system voltage; modules are very sensitive to voltage changes.

•Verify oil, coolant, and fuel levels.

•Inspect for visible leaks.

•If necessary, perform a road test under similar conditions to the customer’s complaint, using information gathered in Step 1 of this procedure.

TECHNICIAN TIP

The technician and customer should both be aware that the repair of a fault code might lead to additional DTCs setting later. On-board diagnostics generation II (OBD II) operation can prevent one or more monitors from running with a stored DTC. Successfully repairing a code allows the powertrain control module (PCM) to run additional tests. If additional monitors do not run, because of a stored DTC, the PCM may not detect and store additional faults until the original DTC has been set. Clearing the DTC(s) after a repair enables the monitors to run again, thus continuing system testing.

Step 5: Check Technical Service Bulletins

Always perform a quick check for TSBs and recalls before starting an in-depth diagnosis. Manufacturers release TSBs frequently, and those TSBs may contain information involving parts updates, service procedures, module-programming updates (reflashing), wiring concerns, or descriptions of normal system operation that instruct the technician that no repair attempt is needed. This step can be a huge time saver when performed before diagnosing, not as a last resort when at a dead-end (FIGURE 2-5).

FIGURE 2-5 Looking at TSBs may potentially save the technician time diagnosing the customer concern. TSBs are common faults that are known to the manufacturer, and the solution is included in the TSB.

TECHNICIAN TIP

DTCs do not always refer to a component failure. Read and understand the code definition and criteria required for setting the DTC. Replacing parts based solely on a DTC may result in a repeat repair. Always perform a thorough diagnosis before replacing parts.

Step 6: Diagnostic Categories

6.A—current DTC, also known as a hard fault: This condition is currently present and is typically the easiest to diagnose. Follow the designated DTC diagnostic flow chart to ensure an efficient repair. DTCs may indicate a system fault, requiring an understanding of system operation to perform the right tests.

6.B—intermittent/history DTC: This fault is often the most difficult to diagnose. It does not occur continuously, and it usually requires a particular set of parameters present to replicate it. Intermittent faults are commonly a result of an electrical issue from connections and wiring, radio frequency interference (RFI), malfunctioning components, or aftermarket accessories (including insurance monitors plugged into the OBDII port). Use of service information combined with technician knowledge and symptoms with the conditions supplied by the customer aid significantly in the repair of intermittent concerns. The following are some of the tests that will help replicate concerns:

• Perform wiggle testing on harnesses and connectors.
• Cycle components through complete temperature extremes.
• Road test the vehicle to induce shock and vibration.
• Apply moisture (with a spray bottle) to components and wiring.

Install the scan tool and test drive the vehicle in record mode after selecting the parameter ID (PID) data that correspond to the fault. Recording in graphing mode is preferred for intermittent concerns.

Freeze-frame and failure records can aid in finding intermittent faults. When a DTC sets, the PCM stores freeze-frame records for the first DTC set. Failure records can store information for five codes. The information provided by freeze-frame data includes when the operating conditions present when the fault occurs: at what load, rpm, operating temperature, and vehicle speed. Operating the vehicle under conditions that are not representative of those that were present when the fault occurred will typically fail to replicate the failure, resulting in lost time and a vehicle that will continue to exhibit the customer’s concerns (FIGURE 2-5).

6.C—Symptom with no DTC: Not all symptoms will store a DTC, so parameters and enabling criteria need to be met to set a code. For example, a component that passes an electrical test may fail intermittently mechanically (binding or sticking), but not store a code. Non-DTC diagnosis can be difficult. Diagnosis involves first replicating the problem and then developing a diagnostic path, complete with testing to isolate the cause. Non-DTC diagnosis requires using a symptom-to-cause diagnostic process. Check service information for OEM-supplied symptom charts or diagnostic trees. To use these diagnostic aids, match the symptom to the fault as closely as possible, and then follow the list of possible causes given.

6.D—no published diagnostics: The technician constructs a diagnostic plan matching the present fault or present concern. Using electrical schematics for power, ground, and splices, and using connectors for modules, inputs, and outputs will assist in the diagnosis. System descriptions and operations are also beneficial, and individual components may activate only during a particular set of criteria. Component location information is also an asset for finding connectors, harnesses, and components exposed to temperature extremes, moisture, and corrosive materials such as road salt, battery acid, oil, or other fluids.

Step 7: Isolate the Root Cause and Reexamine the Concern

Based on the diagnostics the technician performed, they identify the proper repair procedure. In some cases, the component found to be at fault may not be the root cause of the concern. For example, a shorted fuel injector winding increases the amperage flow of the circuit, overheating the PCM transistor (driver), which can result in a failure of the controller. Replacing only the PCM, due to its inability to control the fuel injector, will lead to an expensive repeat repair when the PCM fails again. Proper repair requires that both the fuel injector (cause) and the PCM (result) be replaced, guaranteeing a quality repair.

If the condition cannot be successfully isolated or replicated, return to Step 1 and follow the diagnostic path again. In extreme circumstances, it may be necessary to contact a technical assistance hotline, search for information on the Internet through industry information services such as International Association of Technicians (iATN), via search engines, or from a known-good source.

Step 8: Repair Then Verify the Repair

Performing the necessary repairs after isolating the cause requires verification after the repair. To validate the fix to the system or component, replicate the conditions that caused the concern. After completing the verification, check for DTCs again. Testing for DTCs ensures the repair is successful and has not created any new, additional concerns.

Performing the necessary repairs after isolating the cause requires verification after the repair. To validate the repair made to the system or component, replicate the conditions that caused the concern. After completing the verification, process check for DTCs again to ensure the matter repaired and has not caused any new, additional concerns.

Completing a drive cycle is another method to verify repairs. The drive cycle runs all OBD II monitors, testing emission-related onboard diagnostics (OBD). When all the diagnostics are complete, the systems and OBD II monitors will display “Yes,” “True,” or “Complete.” If the technician is unable to perform the complete drive cycle, notify the customer that the concern, and malfunction indicator lamp (MIL), may return. After the technician has cleared the code(s), the monitors that the PCM suspended while the DTC was present will be able to run, checking their operation (FIGURE 2-6). This may find a fault within another system that is reliant on a particular monitor that didn’t run previously because of the fault.

FIGURE 2-6 Using freeze-frame data and failure records can help guide the technician toward the area in need of repair.

TECHNICIAN TIP

Adaptive strategies for engine and transmission operation will default back to factory settings after disconnecting the battery or after the keep alive memory (KAM) has been reset during service—resulting in the vehicle operating differently until the PCM’s adaptive strategy relearns the customer’s driving habits and adjusts to engine and transmission wear. A repeat may occur if the customer isn’t informed that a relearn procedure is required. Typically after resetting the KAM, also known as the adaptive memory, the vehicle relearns adaptive strategies within 50 to 100 miles (80 to 160 km).

Step 9: Transfer Information to the Repair Order

After finishing the repair, transfer the gathered information to the repair order. This information is supplied to the customer, and at an OEM dealer it is supplied to the manufacturer for payment if the repair is covered under warranty. The information given is the basis for the 3 Cs process. The technician’s comments should allow anyone who reads them to understand the reason for the performed service. These comments should detail the specific testing, the results of all the individual tests done, and the specifications that the testing was based on. This information justifies the charges applied to the vehicle, and it acts as a history in the event that another service visit is required.