On-Board Diagnostics: Notes

NOTE: Engine is managed by a Powertrain Control Module (PCM). PCM diagnostic memory is retained by direct power supply from battery. Memory is not cleared/erased by turning off ignition, but it will be cleared/erased if battery or PCM is disconnected.

CAUTION: If battery has to be disconnected, obtain radio anti-theft code before disconnecting battery.

PCM includes On-Board Diagnostics II (OBD-II) software designed to monitor emissions control system over life of vehicle. On OBD-II, Malfunction Indicator Light (MIL) will only illuminate for emissions system related faults. Most of these faults will not cause driveability problems and would go unnoticed by driver were it not for MIL. When MIL illuminates, a DTC will be stored. PCM may also store DTCs that do not illuminate MIL. These may be DTCs for faults that no longer exist, provisional codes, or for faults that are not emissions system related. DTCs may be retrieved using manufacturer's MUT-II Scan Tool (MB991502) or a compatible generic scan tool. See RETRIEVING DIAGNOSTIC TROUBLE CODES .

PCM monitors several different engine control system circuits. If an abnormal input signal occurs, 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 that cause of failure is necessarily within system. Most faults require that malfunction be detected during 2 different trips before MIL is illuminated. For DTCs that use two-trip detection logic, first occurrence of fault will store a provisional code. Second occurrence of fault will set a DTC and illuminate MIL.

For certain system or component failures, PCM will enter fail-safe function (limp home mode). See FAILURE MANAGEMENT . In fail-safe mode, PCM will substitute default values for a faulty component or system.

Each circuit or system monitored by PCM has particular engine and driving conditions that must be met before PCM will monitor it. These conditions are 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 vehicle under conditions that meet code set criteria in order to verify repair. This is best done by following drive cycle related to DTC. To verify a misfire or fuel trim repair, it is also necessary to operate vehicle in conditions similar (engine speed, engine load, engine temperature, etc.) to those when misfire occurred. These conditions are recorded as freeze frame data. Check freeze frame data before erasing DTCs, as freeze frame data will also be cleared.