STRATEGY-BASED DIAGNOSTICS
The strategy-based diagnostic is a uniform approach to repair all Electrical/Electronic (E/E) systems. The diagnostic flow can always be used to resolve an E/E system problem and is a starting point when repairs are necessary. The following steps will instruct technician on how to proceed with a diagnosis
- Verify customer complaint. To verify customer complaint, technician should know normal operation of system.
- Perform preliminary checks as follows: Conduct a thorough visual inspection. Review service history. Detect unusual sounds or odors. Gather Diagnostic Trouble Code (DTC) information to achieve an effective repair.
- Check bulletins and other service information. This includes videos, newsletters, etc.
- Refer to service information (manual) system check(s).
- Refer to service diagnostics.
NO TROUBLE FOUND
This condition exists when vehicle is found to operate normally. The condition described by customer may be normal. Verify customer complaint against another vehicle that is operating normally. The condition may be intermittent. Verify complaint under conditions described by customer before releasing vehicle.
Re-examine complaint. When complaint cannot be successfully found or isolated, a re-evaluation is necessary. The complaint should be re-verified and could be intermittent as defined in "Intermittents", or could be normal.
After isolating cause, repairs should be made. Validate for proper operation and verify that symptom has been corrected. This may involve road testing or other methods to verify that complaint has been resolved under following conditions
- Conditions noted by customer.
- If a DTC was diagnosed, verify a repair by duplicating conditions present when DTC was set as noted in Failure Records or Freeze Frame data.
Passive & Active Diagnostic Tests
A passive test is a diagnostic test which simply monitors a vehicle system or component. Conversely, an active test, actually takes some sort of action when performing diagnostic functions, often in response to a failed passive test. For example. Exhaust Gas Recirculation (EGR) diagnostic active test will force EGR valve open during closed throttle acceleration and/or force EGR valve closed during a steady state. Either action should result in a change in manifold pressure.
Intrusive Diagnostic Tests
This is any on-board test run by Diagnostic Management System which may have an effect on vehicle performance or emission levels.
Warm-Up Cycle
A warm-up cycle means that engine temperature must reach a minimum of 160° F (70° C) and rise at least 72° F (22° C) over course of a trip.
Diagnostic
When used as a noun, word diagnostic refers to any on-board test run by vehicle's Diagnostic Management System. A diagnostic is simply a test run on a system or component to determine if system or component is operating according to specification. There are many diagnostic, shown in following list
- Misfire
- Oxygen Sensors (O2S)
- Heated Oxygen Sensors (HO2S)
- Exhaust Gas Recirculation
- Catalyst Monitoring
Diagnostic Information
The Diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts are prepared with requirement that vehicle functioned correctly at time of assembly and that there are not multiple faults present.
There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by diagnostic procedures contained in this manual. The language of communicating source of malfunction is a system of diagnostic trouble codes. When a malfunction is detected by control module, a diagnostic trouble code is set and Malfunction Indicator Lamp (MIL) is illuminated.
DTC TYPES
Each Diagnostic Trouble Code (DTC) is directly related to a diagnostic test. The Diagnostic Management System sets DTC based on failure of tests during a trip or trips. Certain tests must fail two consecutive trips before DTC is set. The following are three types of DTCs and characteristics of those codes.
READING DIAGNOSTIC TROUBLE CODES
The procedure for reading diagnostic trouble code(s) is to use a diagnostic scan tool. When reading Diagnostic Trouble Codes (DTCs), follow instructions supplied by tool manufacturer.
DTC Modes
On On-Board Diagnostic (OBD II) passenger cars there are five options available in scan tool DTC mode to display enhanced information available. A description of new modes, DTC Info and Specific DTC, follows. After selecting DTC, following menu appears
- DTC Info
- Specific DTC
- Freeze Frame
- Fail Records (not all applications)
- Clear Info
The following is a brief description of each of sub menus in DTC Info and Specific DTC. The order in which they appear here is alphabetical and not necessarily way they will appear on scan tool.
DTC Information Mode
Use DTC info mode to search for a specific type of stored DTC information. There are seven choices. The service manual may instruct technician to test for DTCs in a certain manner. Always follow published service procedures.
To get a complete description of any status, press "Enter" key before pressing desired F-key. For example, pressing "Enter" then an F-key will display a definition of abbreviated scan tool status.
DTC Status
This selection will display any DTCs that have not run during current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. DTC tests which run and pass will cause that DTC number to be removed from scan tool screen.
Last Test Fail
This selection will display only DTCs that failed last time test ran. The last test may have run during a previous ignition cycle if a type A or type B DTC is displayed. For type C and type D DTCs, last failure must have occurred during current ignition cycle to appear as Last Test Fail.
Test Fail SCC (Test Failed Since Code Clear)
This selection will display all active and history DTCs that have reported a test failure since last time DTCs were cleared. DTCs that last failed more than 40 warm-up cycles before this option is selected will not be displayed.
Specific DTC Mode
This mode is used to check status of individual diagnostic tests by DTC number. This selection can be accessed if a DTC has passed, failed or both. Many OBD II DTC mode descriptions are possible because of extensive amount of information that diagnostic executive monitors regarding each test. Some of many possible descriptions follow with a brief explanation.
The "F2" key is used, in this mode. to display a description of DTC. The "Yes" and "No" keys may also be used to display more DTC status information. This selection will only allow entry of DTC numbers that are supported by vehicle being tested. If an attempt is made to enter DTC numbers for tests which diagnostic executive does not recognize, requested information will not be displayed correctly and scan tool may display an error message. The same applies to using DTC trigger option in Snapshot mode If an invalid DTC is entered, scan tool will not trigger.
Failed Last Test
This message display indicates that last diagnostic test failed for selected DTC. For type A and type B DTCs, this message will be displayed during subsequent ignition cycles until test passes or DTCs are cleared. For type C and type D DTCs, this message will clear when ignition is cycled.
History DTC
This message display indicates that DTC has been stored in memory as a valid fault. A DTC displayed as a History fault may not mean that fault is no longer present. The history description means that all conditions necessary for reporting a fault have been met (maybe even currently), and information was stored in control module memory.
Test Ran and Passed
This message display indicates that selected diagnostic test has done following
- Passed last test.
- Run and passed during this ignition cycle.
- Run and passed since DTCs were last cleared.
If indicated status of vehicle is "Test Ran and Passed" after a repair verification, vehicle is ready to be released to customer.
If indicated status of vehicle is "Failed This Ignition" after a repair verification, then repair is incomplete and further diagnosis is required.
Prior to repairing a vehicle, status information can be used to evaluate state of diagnostic test. and to help identify an intermittent problem. The technician can conclude that although MIL is illuminated, fault condition that caused code to set is not present. An intermittent condition must be cause.
PRIMARY SYSTEM-BASED DIAGNOSTICS
There are primary system-based diagnostics which evaluate system operation and its effect on vehicle emissions. The primary system-based diagnostics are listed below with a brief description of diagnostic function
Oxygen Sensor Diagnosis
The fuel control Oxygen Sensor (02S) is diagnosed for following conditions
- Slow response.
- Response time (time to switch R/L or L/R).
- Inactive signal (output steady at bias voltage approx. 450 mv).
- Signal fixed high.
- Signal fixed low.
The catalyst monitor Heated Oxygen Sensor (HO2S) is diagnosed for following conditions
- Heater performance (time to activity on cold start).
- Signal fixed low during steady state conditions or power enrichment (hard acceleration when a rich mixture should be indicated).
- Signal fixed high during steady state conditions or deceleration mode (deceleration when a lean mixture should be indicated).
- Inactive sensor (output steady at approximately 438 mv).
If oxygen sensor pigtail wiring, connector or terminal are damaged, entire oxygen sensor assembly must be replaced. Do not attempt to repair wiring, connector or terminals. In order for sensor to function properly, it must have clean reference air provided to it. This clean air reference is obtained by way of oxygen sensor wire(s). Any attempt to repair wires, connector or terminals could result in obstruction of reference air and degrade oxygen sensor performance.
Fuel Trim Cell Diagnostic Weights
No fuel trim DTC will set regardless of fuel trim counts in cell 0 unless fuel trim counts in weighted cells are also outside specifications. This means that vehicle could have a fuel trim problem which is causing a problem under certain conditions (i.e engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or H02S DTC may set). Use a scan tool to observe fuel trim counts while problem is occurring.