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.
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HOMESERVICE MANUALSJEEP1991CHEROKEE 2WD L4-150 2.5L VIN P MFIREPAIR AND DIAGNOSISCRUISE CONTROLTESTING AND INSPECTIONCOMPONENT TESTS AND GENERAL DIAGNOSTICSON-BOARD DIAGNOSTIC SYSTEMOVERVIEW
1991 Jeep Cherokee 2WD L4-150 2.5L VIN P MFI
On-Board Diagnostic System: Overview
1991 Jeep Cherokee 2WD L4-150 2.5L VIN P MFISECTION Overview
The speed control and charging systems are monitored by the engine controller. The engine controller monitors critical input and output circuits within these systems, making sure they are operating correctly.
Some circuits are tested continuously; others are checked only under certain circumstances.
Each circuit monitored by the engine controller has a corresponding fault message assigned to it. Refer to "System Fault Messages" later in this section for a list and definition of the fault messages.
- If the on-board diagnostic system senses that one of the circuits is malfunctioning, it stores the corresponding fault message in memory.
- If the malfunction goes away after the fault is stored, the fault message will be erased after 50 key cycles.
Some circuits are tested continuously; others are checked only under certain circumstances.
Each circuit monitored by the engine controller has a corresponding fault message assigned to it. Refer to "System Fault Messages" later in this section for a list and definition of the fault messages.
- If the on-board diagnostic system senses that one of the circuits is malfunctioning, it stores the corresponding fault message in memory.
- If the malfunction goes away after the fault is stored, the fault message will be erased after 50 key cycles.
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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.