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: Notes
Malfunction Indicator Light (MIL) may also be referred to as CHECK ENGINE light. MIL is located on lower right side of instrument cluster and is represented by an engine symbol. When ignition is first turned on, MIL should come on and remain on for 2 seconds to verify bulb and circuit operation, and then go off. If MIL does not come on at all, check instrument cluster for diagnosis of MIL. See INSTRUMENT CLUSTER SYSTEMS under SYSTEM TESTS in BODY CONTROL MODULES - CONCORDE, INTREPID, LHS & 300M article in ACCESSORIES & EQUIPMENT.
Powertrain Control Module (PCM) monitors several different engine control system circuits. If a malfunction occurs which affects vehicle emissions, a Diagnostic Trouble Code (DTC) will be stored in PCM memory and PCM will enter limp-in mode. In limp-in mode, PCM substitutes values for the failed component to continue engine operation, but a loss of good driveability may result. When DTC is stored in PCM memory, PCM will turn MIL on and MIL will remain on steady. If problem exists which does not affect vehicle emissions, a DTC will be stored in PCM but MIL will not be turned on.
PCM might not store a DTC for a monitored circuit even though a malfunction has occurred. This may happen because one of the criteria for the DTC has not been met. Criteria may be a specific engine RPM range, engine temperature and/or voltage input to PCM. For example, assume criteria for DTC requires PCM to monitor a specified sensor output circuit only when engine operates at 750-2000 RPM. If sensor output circuit becomes shorted to ground when engine is operating at 2400 RPM, DTC will not be stored, as short to ground occurred at 2400 RPM which exceeds the limit when PCM monitors the circuit.
If PCM detects an active engine misfire severe enough to cause catalytic converter damage, MIL will immediately start flashing to warn driver of possible catalytic converter damage and DTC will be stored. PCM may turn off MIL if malfunction is not detected during 3 consecutive trips, 3 consecutive engine misfire monitor tests or fuel system monitor tests. For additional information on good trips, see TRIP INDICATOR . For additional information on engine misfire monitor and fuel system monitor, see MONITORED CIRCUITS . PCM performs engine misfire and fuel system monitor tests within predetermined engine speed (RPM) and load operating conditions.
DTCs may only be retrieved for system diagnosis by using a scan tool. See RETRIEVING DIAGNOSTIC TROUBLE CODES . System malfunctions are identified as either hard failures or intermittent failures. For additional information on hard failures or intermittent failures, see 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.