Description of On-Board Diagnostics
OVERVIEWThe Powertrain Control Module (PCM) monitors many different circuits in the fuel injection, ignition, emission and engine systems. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a Diagnostic Trouble Code (DTC) in the PCM's memory. If the code applies to a non-emissions related component or system and the problem is repaired or ceases to exist, the PCM cancels the code after 40 warm-up cycles. Diagnostic trouble codes that affect vehicle emissions illuminate the Malfunction Indicator Lamp (MIL).
PURPOSE
The on-board diagnostics incorporated with the Powertrain Control Module (PCM) are intended to assist the field technician in repairing vehicle problems by the quickest means.
The PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "On-board Diagnosis".
NEW ELECTRONIC CIRCUIT MONITORS
There are new electronic circuit monitors that check fuel, emission, engine and ignition performance. These monitors use information from various sensor circuits to indicate the overall operation of the fuel, engine, ignition and emission systems and thus the emissions performance of the vehicle.
The fuel, engine, ignition and emission systems monitors do not indicate a specific component problem. They do indicate that there is an implied problem within one of the systems and that a specific problem must be diagnosed.
If any of these monitors detect a problem effecting vehicle emissions the Malfunction Indicator (Check Engine) Lamp will be illuminated. These monitors generate Diagnostic Trouble Codes that can be displayed with the check engine lamp or a scan tool.
OPERATION
Certain criteria (arming conditions) must be met for a trouble code to be set in PCM memory. The criteria may include: engine rpm, engine temperature, and/or PCM input voltage. If a problem is sensed in a monitored circuit, and all of the criteria (arming conditions) are met, a trouble code will be stored in the PCM.
It is possible that a trouble code for a monitored circuit may not be entered into the PCM memory even though a malfunction has occurred. This may happen because one of the trouble code arming conditions has not been met. For example, assume the diagnostic trouble code criteria requires the PCM to monitor the circuit only when the engine operates between 750 and 2000 RPM. Suppose the sensor's output circuit shorts to ground when engine operates above 2400 RPM (resulting in 0 volt input to the PCM). Because the condition happens at an engine speed above the maximum threshold (2000 RPM), the PCM will not store a DTC.
The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code arming conditions are met, a trouble code will be set in PCM memory.
There are several operating conditions for which the PCM monitors and sets DTC's. Refer to Monitored Circuits, and Non-Monitored Circuits in this section.
Various diagnostic procedures may actually cause a diagnostic monitor to set a DTC. For instance, pulling a spark plug wire to perform a spark test may set the misfire code. When a repair is completed and verified, use the DRB scan tool to erase all DTC's and extinguish the MIL.
MALFUNCTION INDICATOR LAMP (MIL)
As a functional test, the Malfunction Indicator Lamp (MIL) illuminates at key-on before engine cranking. Whenever the Powertrain Control Module (PCM) sets a Diagnostic Trouble Code (DTC) that affects vehicle emissions, it illuminates the MIL.
The PCM illuminates the MIL only for DTC's that affect vehicle emissions.
The MIL stays on continuously, when the PCM has entered a Limp-In mode or identified a failed emission component. The MIL remains on until the DTC is erased.
Also, the MIL either flashes or illuminates continuously when the PCM detects active engine misfire.
Additionally, the PCM may reset (turn OFF) the MIL when one of the following occur:
- PCM does not detect the malfunction for three consecutive trips.
- PCM does not detect a malfunction while performing three successive engine misfire or fuel system tests. The PCM performs these tests while the engine operates within 375 RPM, 10 percent load, and the same warm-up status in which the original malfunction was detected.
Hard Codes
A hard code is a code that comes back within one cycle of the ignition key. This means the defect is there every time the PCM checks that circuit or function. Procedures included here verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an intermittent test must be performed.
Intermittent Codes
A diagnostic trouble code that is not there every time the powertrain control module checks the circuit is an "intermittent" code.
Most intermittent codes are caused by wiring or connector problems. Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them.
Try to reproduce the conditions that cause the intermittent condition to occur.
Reset Counter
The reset counter counts the number of times the vehicle has been started since codes were last set, erased, or the battery was disconnected. The reset counter will count up to 255 start counts.
The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on a suitable scan tool.
When there are no trouble codes stored in memory, the scan tool will display "No trouble codes found" and the reset counter will show "Reset Count = XXX".
STATE DISPLAY TEST MODE
The switch inputs to the Powertrain Control Module (PCM) have two recognized states; HIGH and LOW. For this reason, the PCM cannot recognize the difference between a selected switch position versus an open circuit, a short circuit, or a defective switch. If the State Display screen shows the change from HIGH to LOW or LOW to HIGH, assume the entire switch circuit to the PCM functions properly. From the state display screen, access either State Display Inputs and Outputs or State Display Sensors.
CIRCUIT ACTUATION TEST MODE
The Circuit Actuation Test Mode checks for proper operation of output circuits or devices the Powertrain Control Module (PCM) may not internally recognize. The PCM attempts to activate these outputs and allow an observer to verify proper operation. Most of the tests provide an audible or visual indication of device operation (click of relay contacts, fuel spray, etc.). Except for intermittent conditions, if a device functions properly during testing, assume the device, its associated wiring, and driver circuit work correctly.