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.
Engine Controls - Self-Diagnostics - Introduction: Introduction: OBD-II
ECM detects malfunctions related to engine sensors or actuators. OBD-II monitors emission related components and can store Diagnostic Trouble Codes (DTCs), 1st trip DTCs, freeze-frame data, 1st trip freeze-frame data and System Readiness Test (SRT) information. Using a scan tool, this data and DTC information can be retrieved to assist in diagnosing problems. On all models except Maxima, Quest and Sentra, the Malfunction Indicator Light (MIL) can be used for code retrieval and fuel feedback monitoring.
When a malfunction is detected for the first time, the 1st trip DTC and 1st trip freeze frame data are stored in Engine Control Module (ECM) or Powertrain Control Module (PCM) memory. However, in most instances, MIL will not be illuminated. If the same malfunction is detected on the next trip (under similar conditions), MIL will be illuminated and a DTC is stored (2nd trip detection logic). If a misfire or fuel system malfunction should occur during 1st trip, MIL may be illuminated or flash. DTC and freeze-frame information will be stored to aid in diagnostics by specifying operating conditions when DTC was set.
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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.
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