Flow of Diagnosis
Flow Of Diagnosis:
1. Verify that the problem exist and verify whether the problem is a intermittent or a constant problem. DO NOT USE TROUBLE CODE CHARTS for diagnosing of intermittent problems. NOTE: For further information, use the DRIVEABILITY WORKSHEET and the VEHICLE REPAIR HISTORY forms below.
2. Check to make sure vacuum and vapor hose routing is correct and that hoses are not split or cracked. Check to make sure all the electrical connections to sensors and solenoids are clean and have tight connections. Check to make sure all engine grounds are clean and have good connections.
3. Check for updated diagnostic procedures warranty or recall information.
4. Perform the DIAGNOSTIC CIRCUIT CHECK under TESTING PROCEDURES. This checks to make sure that the computer is functioning properly and that the "SERVICE ENGINE SOON LIGHT" is working. This is the starting point for the diagnosis of the problem, and this circuit check will direct you to the proper test procedure.
VISUAL/PHYSICAL UNDERHOOD INSPECTION
A careful visual and physical underhood inspection must be performed as part of any diagnostic procedure or in finding the cause of a failure. This can often lead to fixing a problem without further steps. Inspect all vacuum hoses for correct routing, pinches, cuts or disconnects. Be sure to inspect hoses that are difficult to see beneath the air cleaner, compressor, generator, etc. Inspect all the wires in the engine compartment for proper connections, burned or chafed spots, pinched wires, or contact with sharp edges or hot exhaust manifolds. This visual/physical inspection is very important. It must be done carefully and thoroughly.
DIAGNOSTIC INFORMATION
The diagnostic trouble tree charts and functional checks are designed to locate a faulty circuit or component through logic based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are no multiple failures. Trouble tree charts are designed to be used only when a code or flag is present.
The PCM performs a continual self diagnosis on certain control functions. The PCM's language for communicating the source of a malfunction is a system of diagnostic trouble codes. The codes are two digit numbers that can range from 11 to 99. When a malfunction is detected by the PCM, a code is set and the SERVICE ENGINE SOON telltale lamp is illuminated.
^ Code: Suspected failure detected in the engine or transaxle.
^ Flag: A diagnostic aid used when a code or intermittent problem occurs. It does not necessarily indicate a failure.
SERVICE ENGINE SOON TELLTALE LAMP
This telltale lamp is in the instrument cluster and has the following functions:
^ It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
^ It displays trouble codes stored by the PCM which help the technician diagnose system problems.
^ It indicates Open Loop or Closed Loop engine operation.
As a bulb and system check, the lamp will come "ON" with the key "ON" and the engine not running. When the engine is started, the lamp will turn "OFF". If the lamp remains "ON", the self diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases after 10 seconds, but a trouble code will remain stored in the PCM.
When the lamp remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability problem a Diagnostic Circuit Check must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.
INTERMITTENT SERVICE ENGINE SOON TELLTALE LAMP OPERATION
In the case of an intermittent problem, the SERVICE ENGINE SOON telltale lamp will light for 10 seconds and then will go out. However, the corresponding trouble code or flag will be stored in the memory of the PCM until removed.
An intermittent problem may or may not set a code or flag. If it is an intermittent failure, a Diagnostic Trouble Code Chart is not used. Consult Diagnostic Charts. A physical inspection of the applicable sub system most often will resolve the problem.
DIAGNOSTIC CIRCUIT CHECK
The diagnostic circuit check is always the starting point for any malfunction code or driveability problem. This check verifies the PCM's on board diagnostics are functioning properly. While performing a diagnostic circuit check, record codes and flags for use in diagnosing possible sub system failures.