System Diagnosis: Notes

CAUTION: Obtain radio anti-theft code before disconnecting battery.

NOTE: Engine management is controlled by an Engine Control Module (ECM) on vehicles with Manual Transaxle (M/T) or by a Powertrain Control Module (PCM) on vehicles with Automatic Transaxle (A/T). 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.

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 equipped vehicles, the MIL will illuminate only 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 DTC will be stored. The ECM/PCM may also store DTCs that do not illuminate the MIL. These may be DTCs for faults that no longer exist, provisional codes or for faults that are not emission systems related. DTCs may be retrieved using the manufacturer's MUT-II Scan Tool (MB991502) or a compatible generic scan tool. See ENTERING ON-BOARD DIAGNOSTICS under SELF-DIAGNOSTIC SYSTEM.

ECM/PCM monitors several different engine control system circuits. If an abnormal input signal occurs, the ECM/PCM will store it in memory as either a provisional code or DTC. A specific DTC indicates a particular system failure, but does not indicate the cause of failure is necessarily within that system. Most faults require that the malfunction be detected during 2 different drive cycles before the MIL is illuminated. On DTCs that use 2 drive cycles, the first occurrence of the fault will store a provisional code. The second occurrence of the fault will set a DTC and illuminate the MIL.

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. This is best done by following the drive cycle related to the DTC. 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.