On-Board Diagnostics

The Powertrain Control Module (PCM) monitors several different engine control system circuits. If PCM detects a malfunction on a monitored circuit often enough to indicate a problem, a Diagnostic Trouble Code (DTC) will be stored in PCM memory.

If DTC is for a non-emissions related system or component, and problem is repaired or does not reoccur, PCM cancels DTC after 40 consecutive trips. PCM will cancel DTC's which affect emissions (engine misfire and fuel system monitor) after 80 consecutive trips if malfunction does not reoccur.

Excessive engine misfires result in increased catalyst temperature and cause an increase Hydrocarbon (HC) emissions. Severe misfires may cause catalyst damage. PCM monitors for misfire during most engine operating conditions by observing crankshaft speed. If a misfire occurs, crankshaft speed will vary more than normal.

For fuel system monitor, PCM makes short term adjustments in injector pulse width based on O2S input. PCM uses programmed memory to act like a self calibration tool to compensate for engine specification variations, engine wear and sensor tolerances over useful life of engine. PCM monitors actual air/fuel ratio with O2S. PCM multiplies O2S (short term) with programmed (long term) adaptive memory, and compares sum with limit. PCM can determine if vehicle will pass an emissions test. If PCM cannot maintain optimum air/fuel ratio, MIL will be illuminated.

The PCM contains electronic circuit monitors that monitor fuel, vehicle emissions, engine and ignition system performance. Monitors do not indicate a specific component failure, but do indicate an implied failure within a specified system, and that the problem must be diagnosed. If any monitor detects a problem affecting vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM and MIL will be illuminated. The following monitors are used:

• EGR Monitor
• Engine Misfire Monitor
• Fuel System Monitor
• Oxygen Sensor Monitor
• Oxygen Sensor Heater Monitor
• Catalyst Monitor
• EVAP System Leak Detection Monitor

If PCM does not detect the malfunction for 3 consecutive trips, except for misfire and fuel system monitors. For misfire and fuel system monitors, if PCM does not detect the malfunction while engine is operating within 375 RPM of and within 10 percent of load of operating condition when malfunction was first detected.

A trip is defined as the engine is started, an increase of 40°F (4°C) in engine coolant temperature exists after engine is started, and engine coolant temperature reaches at least 160°F (71°C).

The PCM also records and stores engine operating conditions when malfunction occurred. This information is referred to as freeze frame data. If malfunction is an engine misfire or fuel system rich or fuel system lean, freeze frame data will be updated with the most current information regarding these failures.

Freeze frame data recorded is:

• Fuel System Status
• Load Value (Displayed As Percent)
• Engine Coolant Temperature
• Short Term Adaptive (Displayed As Percent)
• Long Term Adaptive (Displayed As Percent)
• Absolute MAP (Displayed As Percent)
• Engine RPM
• Vehicle Speed Sensor
• Diagnostic Trouble Code (DTC) During Data Recording

DTCs may be retrieved for system diagnosis. See procedures under RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. System malfunctions are identified as either hard failures or intermittent failures.