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 - Basic Diagnostic Procedures - Gasoline: Introduction
The following diagnostic steps help prevent overlooking simple problems. This is also where to begin diagnosis for no-start condition.
The first step in diagnosing any driveability problem is verifying the customer's complaint by test driving vehicle under the conditions in which the problem reportedly occurred.
Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed.
The following table provides location of commonly used diagnostic information. These former "A" and "C" charts are now written in text and inserted into appropriate location of engine performance workflow. To learn about engine performance workflow, see HOW TO USE THE ENGINE PERFORMANCE SECTION in this section.
| System Or Component | Diagnostic Information Location | ||||||
|---|---|---|---|---|---|---|---|
| A-1, Malfunction Indicator Light (MIL) | See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK | ||||||
| A-2, DLC & MIL On Steady | See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK | ||||||
| A-2, No Scan Tool Data | See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK | ||||||
| A-3, No-Start Diagnosis | See A-3, NO-START DIAGNOSIS | ||||||
| A-4, Injector Circuit Diagnosis | See appropriate BASIC FUEL SYSTEM CHECKS | ||||||
| A-5, Fuel Pump Relay | See RELAYS, SOLENOIDS, MOTORS & MODULES in SYSTEM/COMPONENT TESTS article | ||||||
| A-6, Fuel System Diagnosis | See appropriate BASIC FUEL SYSTEM CHECKS | ||||||
| A-7, Injector Balance Test | See FUEL SYSTEM in SYSTEM/COMPONENT TESTS article | ||||||
| C-1, MAP Sensor | See ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article | ||||||
| C-1, Power Steering Pressure Switch | See ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article | ||||||
| C-1, Park/Neutral Switch | See ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article | ||||||
| C-2, IAC Motor | See IDLE CONTROL SYSTEM under FUEL SYSTEM in SYSTEM/COMPONENT TESTS article | ||||||
| C-3, Fuel Evaporation Control | See appropriate EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM/COMPONENT TESTS article | ||||||
| C-4, Ignition Control Circuit | See IGNITION SYSTEM in SYSTEM/COMPONENT TESTS article | ||||||
| C-5, ESC Ignition Check | See IGNITION SYSTEM in SYSTEM/COMPONENT TESTS article | ||||||
| C-7, EGR System | See appropriate EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM/COMPONENT TESTS article | ||||||
| C-8, Torque Converter Clutch | (1) See MISCELLANEOUS PCM/VCM CONTROLS in SYSTEM/COMPONENT TESTS article | ||||||
| C-8, Manual Transmission Shift Lights | (2) See MISCELLANEOUS PCM/VCM CONTROLS in SYSTEM/COMPONENT TESTS article | ||||||
| C-10, A/C Clutch Control | See MISCELLANEOUS PCM/VCM CONTROLS in SYSTEM/COMPONENT TESTS article | ||||||
| C-12, Electric Cooling Fan Control | (3) See MISCELLANEOUS PCM/VCM CONTROLS in SYSTEM/COMPONENT TESTS article | ||||||
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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.