Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
On-Board Diagnostics
Powertrain Control Module (PCM) monitors several different engine control system circuits. If malfunction occurs, PCM will store a Diagnostic Trouble Code (DTC) when malfunction is detected, and PCM will enter limp-in mode. In limp-in mode, PCM substitutes values for failed component to continue engine operation, but loss of good driveability may result.
PCM contains electronic circuit monitors that monitor fuel, vehicle emissions, engine and ignition system performance. Monitors use information from various sensor circuits for system monitoring. Monitors do not indicate a specific component failure, but indicate an implied failure within a specified system, and that problem must be diagnosed. If any monitor detects a problem affecting vehicle emissions, a DTC will be stored in PCM. The following monitors are used:
- Engine Misfire Monitor
- Fuel System Monitor
- Oxygen Sensor Monitor
- Oxygen Sensor Heater Monitor
- Catalyst Monitor
- EVAP System Leak Detection Monitor
MIL may be turned off by PCM, however, a DTC may remain stored depending on type of malfunction detected. To erase DTCs, see CLEARING DTCS under SELF-DIAGNOSTIC SYSTEM. Once MIL is turned off, PCM must not detect recent malfunction during 40 run/stop engine cycles with engine coolant temperature at least 160°F (71.1°C) on normal operation, or 80 run/stop engine cycles with engine coolant temperature at least 160°F (71.1°C) on engine misfire or fuel system monitor.
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 most current information regarding these failures. Freeze frame data recorded is:
- Fuel System Status (Closed Or Open Loop)
- Load Value (Displayed As Percent)
- Engine Coolant Temperature
- Short Term Adaptive (Displayed As Percent)
- Long Term Adaptive (Displayed As Percent)
- MAP Vacuum
- Engine RPM
- Vehicle Speed Sensor
- DTC Causing Freeze Frame During Data Recording
- Freeze Frame Priority
DTCs may be retrieved for system diagnosis by using scan tool only. See RETRIEVING DTCS under SELF-DIAGNOSTIC SYSTEM. By using scan tool, self-diagnostic capabilities of this system can simplify testing and reduce diagnostic time. Erasing DTCs also erases freeze frame data. See CLEARING DTCS under SELF-DIAGNOSTIC SYSTEM. System malfunctions are identified as either hard failures or intermittent failures.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.