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HOMESERVICE MANUALSBUICK1999REGAL LSREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P0205-P0351DTC P0303DTC P0303: CYLINDER NO. 3 MISFIRE DETECTED (3.0L "V" BODY)DIAGNOSTIC PROCEDURES
1999 Buick Regal LS

Diagnostic Procedures

1999 Buick Regal LSSECTION Diagnostic Procedures
  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
  2. Start and idle engine. Using scan tool, monitor misfire current counter for this cylinder. If misfire current counter increases, go to next step. If any misfire current counter does not increase, go to step  4.
  3. Inspect vacuum hoses for splits, kinks and proper connections. Check throttle body and intake manifold for vacuum leaks. Check PCV valve operation and system for vacuum leaks. Ensure ECM grounds are clean and secure. Ensure all injector harness connectors and spark plug wires are installed at correct locations and secure. Repair as necessary. After repairs, go to step  8. If all components and systems are okay, go to step  5.
  4. Turn ignition on, with engine off. Review FREEZE FRAME data for this DTC and note parameters. Clear DTCs. Turn ignition off for 15 seconds. Start engine and operate vehicle as close to conditions recorded in FREEZE FRAME as possible. Check for DTCs. If DTC P0303 is set, go to next step. If DTC P0303 is not set, see DIAGNOSTIC AIDS .
  5. Using Spark Plug Tester (J-26792), check spark at affected cylinder while cranking engine. If spark is present, go to next step. If spark is not present, diagnose ignition system. See BASIC IGNITION SYSTEM CHECKS in appropriate BASIC DIAGNOSTIC PROCEDURES article.
  6. Remove spark plug(s) as necessary and inspect for signs of oil or coolant fouling. Replace spark plug(s) or repair engine as necessary. If spark plugs are okay, go to next step.
  7. Check spark plugs for cracks or damage, excessive wear or improper gap. Check for carbon tracking on insulator. Replace components as necessary. After repairs, go to step  9. If components are okay, go to next step.
  8. Check compression and perform leak down test on affected cylinder. Replace components as necessary. After repairs, go to next step. If components are okay, diagnose fuel system malfunction. See BASIC FUEL SYSTEM CHECKS in appropriate BASIC DIAGNOSTIC PROCEDURES article.
  9. Using scan tool, clear DTCs. Turn ignition off for 15 seconds. Start engine and operate vehicle. Check for DTCs. If DTC P0303 is set, go to step  2. If DTC P0303 is not set, go to next step.
  10. If other DTCs were present during OBD system check and have not been diagnosed, go to applicable DTC. If no other DTCs are present, system is okay.
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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.