System Diagnosis: Notes

NOTE: Some models may use an Engine Control Module (ECM) or a Powertrain Control Module (PCM). In testing procedures and illustrations, the terms may be used interchangeably. For correct application, see WIRING DIAGRAMS article. Engine Control Module/Powertrain Control Module (ECM/PCM) diagnostic memory is retained by direct power supply from battery. Memory is not erased by turning off ignition, but it will be erased if battery or ECM/PCM is disconnected.

NOTE: Some ECM/PCM data and tests are only accessible with manufacturer's MUT-II scan tool. Those tests are not covered in this article. All codes that turn on MIL can be retrieved with compatible generic scan tool. Those tests are covered in this article.

ECM/PCM includes On Board Diagnostics II (OBD-II) software designed to monitor emissions control system over the life of the vehicle. On OBD-II, the Malfunction Indicator Light (MIL) (Service Engine Soon light) will only illuminate for emissions system related faults. Most of these faults will not cause driveability problems and would go unnoticed by the driver were it not for the MIL. When MIL illuminates, a Diagnostic Trouble Code (DTC) will be stored. The ECM/PCM may also store DTCs that do not illuminate the MIL. These may be pending codes, or are not emissions system related. On all models, DTCs may be retrieved using a MUT-II or a compatible generic scan tool. On Eclipse 2.0L non-turbo, the MIL can also be used. See ENTERING ON-BOARD DIAGNOSTICS .

ECM/PCM monitors several different engine control system circuits. If an abnormal input signal occurs, a Diagnostic Trouble Code (DTC) is stored in ECM/PCM memory as either a pending or mature code. A specific DTC indicates a particular system failure, but does not indicate that cause of failure is necessarily within system. Most faults require that the malfunction be detected during 2 different trips before the MIL is illuminated. The first occurrence of the fault will store a pending DTC. The second occurrence of the fault will set a DTC and illuminate the MIL. Faults that will cause catalytic converter damage will turn on the MIL when the malfunction is first detected. In the case of severe misfire, MIL will flash on and off as long as misfire persists.

For certain system or component failures, ECM/PCM will enter fail-safe function (limp home mode). In fail-safe mode, ECM/PCM will substitute default values for the faulty component or system.

Each circuit or system monitored by ECM/PCM has particular engine and driving conditions that must be met before ECM/PCM will monitor it. These conditions are the code set criteria. Due to differences in code set criteria, some circuits or systems are monitored almost continuously while others may not be monitored at all during a particular drive cycle. After a repair, it is necessary to run the vehicle under conditions that meet code set criteria in order to verify the repair. To verify a misfire or fuel trim repair, it is also necessary to operate the vehicle in conditions similar (engine speed, engine load, engine temperature, etc.) to those when the misfire occurred. These conditions are recorded as freeze frame data. Check freeze frame data before erasing DTCs, as freeze frame data will also be erased.