On-Board Diagnostic System

How To Read Diagnostic Trouble Code:

PRINCIPLES OF CODE CYCLE
There are two types of diagnostic trouble codes that can be received. The first code numbers retrieved will be present failures. Present failures are faults that are still occurring. After all present failure diagnostic trouble codes are received, the separator code (10) will be received. The separator code separates present failure diagnostic trouble codes from stored failure diagnostic trouble codes. After the separator codes are received, the stored failures will be received. Stored failures are faults that occurred within 80 key cycles.

NOTE: During the Key On Engine Running Inspection Procedure, only present failures are received.

KEY ON ENGINE OFF INSPECTION
The Key On Engine Off Inspection Procedure is performed to inspect the engine control system components with power supplied to the inputs and outputs but without the engine running.

There are three different types of codes received during the Key On Engine Off Inspection Procedure. The first code received is a diagnostic trouble code for present failures. These are malfunctions that are present at the time of testing. The second type of code received is a separator code. This code is used to separate the present failure diagnostic trouble codes from the stored diagnostic trouble codes. The last type of code received is the stored diagnostic trouble codes. These are malfunctions that occurred while the vehicle was in use but, may or may not still be present. The stored diagnostic trouble codes are kept in memory for 80 key cycles unless erased.

KEY ON ENGINE RUNNING INSPECTION
The Key On Engine Running Inspection Procedure is performed to inspect the engine control system components under normal operating conditions. The outputs are activated and checked for expected results.

There are two types of codes received during the Key On Engine Running Inspection Procedure. The first code received will be an engine ID code. This code represents the number of cylinders of the engine. One beep will be heard for two cylinders. For example, two beeps indicate a four cylinder engine. This code is used to verify the PCM in the vehicle is correct. The other code received is a diagnostic trouble code for present failures.

DYNAMIC RESPONSE TEST
The Dynamic Response Test is performed to verify the operation of the throttle position sensor and the mass air flow sensor during wide-open throttle operation.

CYLINDER BALANCE TEST
The Cylinder Balance Test is performed to assist in detecting a weak or dead cylinder. Once the test is entered, the idle air control solenoid is fixed and the engine is allowed to stabilize. The Powertrain Control Module monitors engine rpm and stores the information. The fuel injector is shut off at the number 6 cylinder. After the engine stabilizes, the PCM records the engine rpm. Once this value is stored the fuel injector at the number 6 cylinder is allowed to operate. This process is repeated for each of the cylinders. After all cylinders are completed, the PCM determines the maximum rpm drop. The maximum rpm drop is multiplied by the calibratable percentage. The result is used as a minimum rpm drop that each cylinder must have dropped to pass the test.

If all the cylinders drop at least the minimum rpm drop, a 90 code will be received indicating all of the cylinders pass the test. No further testing is necessary. If a cylinder did not drop at least the minimum rpm drop, the cylinder number code would be received. This indicates the cylinder is either weak or dead. The Cylinder Balance Test can be repeated to determine if the cylinder is weak or dead. Each time the test is repeated the minimum rpm drop is reduced. If the cylinder fails the test more than three times, the cylinder is dead.

OUTPUT STATE DIAGNOSTIC TEST
The Output State Diagnostic Test is performed to allow the technician to activate the outputs of the PCM.

WIGGLE TEST
The Wiggle Test is performed by the technician in an attempt to re-create an intermittent fault.