GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2001 🢒 Regal 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC P0300

System Description

The powertrain control module (PCM) uses information from the ignition control (IC) module and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring the variations in the crankshaft rotation speed for each cylinder the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause the three-way catalytic converter to overheat under certain conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for three-way catalytic converter overheating are present.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0125, P0336, P0341, P0502, P0503, P1106, P1107, P1114, P1115, P1121, P1122, P1336, P1351, P1352, P1361, P1362, or P1374 are not set.
    • The engine speed is at least 525 RPM and is no more than 5900 RPM.
    • The system voltage between 9-18 volts.
    • The engine coolant temperature (ECT) is at least -7°C (21°F) and is no more than 125°C (255°F).
    • The throttle angle is steady.

Conditions for Setting the DTC

The PCM is detecting a crankshaft rotation speed variation indicating a misfire rate sufficient to cause emissions levels to exceed mandated standard.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • The scan tool will indicate if a test has failed since the codes have been cleared. Misfire history will be stored and displayed for each of the cylinders at the time of the failure.
    • Incorrect rough road received from the EBCM or the EBTCM.
    • The crankshaft position (CKP) system variation learn procedure must be performed if any of the following conditions are true:
       - The PCM has been replaced.
       - DTC P1336 is set.
       - The engine has been replaced.
       - The crankshaft has been replaced.
       - The crankshaft balancer has been replaced.
       - The crankshaft position sensor has been replaced.
    • If the condition is intermittent, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Depending on the cause of the misfire, the Misfire History Cyl # counter will display a very large number for the misfiring cylinders. Values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinders. When investigating a misfire, always start with items associated with the cylinders that has the largest number of counts stored in the Misfire History Cyl # counter.

    2.

  2. If the misfiring cylinders are companion cylinders, the condition is most likely linked to the ignition system.

    3.

  3. Inspect for poor terminal connection, grooves, corrosion, pitting, loose fit.

    4.

  4. Inspect for the following conditions that may contribute to the engine misfire: Engine oil pressure, damaged accessory drive belt or pulley, damaged driven accessory, e.g. generator, water pump, drive belt tensioner, etc., and loose or broken motor mounts.

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Start and idle the engine.
  2. Review and record scan tool Freeze Frame/Failure Records data.
  3. Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame/Failure Records data).
  4. Monitor the scan tool Misfire Current Cyl # display for each cylinder.

Is Misfire Current # display incrementing for any cylinder (indicating a misfire currently occurring)?

--

Go to Step 5

Go to Step 4

4

View the Misfire History Cyl # display on the scan tool.

Does Misfire History Cyl # display a very large value for more than one cylinder?

--

Go to Step 5

Go to Step 8

5

Are the misfire values displayed on the Misfire History Cyl # related to companion cylinders (i.e., 1/4, 2/5, 3/6)?

--

Go to Step 9

Go to Step 6

6

Visually and physically inspect the following items:

    • Excessive vibration from sources other than the engine. The drive belts--Refer to Symptoms - Engine Mechanical in Engine Mechanical.
    • The vacuum hoses--Refer to Emission Hose Routing Diagram .
    • The crankcase ventilation system for vacuum leaks
    • The throttle body and intake manifold for vacuum leaks
    • The PCM grounds--Refer to Power Distribution Schematics in Wiring Systems.
    • The spark plug wires--Refer to Spark Plug Wire Inspection .
    • Measure the resistance of the spark plug wires associated with the misfiring cylinders. Refer to Ignition System Specifications .
    • The EGR valve to exhaust manifold pipe for proper connections and signs of damage
    • The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 18

Go to Step 7

7

  1. Install a fuel pressure gauge.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the fuel pump ON.
  4. Monitor the fuel pressure with the fuel pump running.

Does the fuel pressure gauge indicate that the fuel pressure is within the specified range?

358-405  kPa (52-59 psi)

Go to Step 8

Go to Fuel System Diagnosis

8

Test for proper fuel injector operation. Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct the condition?

--

Go to Step 18

Go to Step 9

9

  1. Visually inspect the ignition wires and coils associated with the misfiring cylinders for the following conditions:
    2. •  Carbon tracking/arching and damage.
    •  Connected to the incorrect cylinders at the coil and spark plug.
    •  Poor terminal connections at the coil and spark plug.
  2. If a condition is found, repair as necessary.

Important: :  If carbon tracking is apparent at either end of the spark plug wires, replace the affected ignition wire and the associated ignition coil.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 10

10
  1. Install the J 26792 Spark Tester at the spark plug end of the ignition wire associated with the misfiring cylinders.
  2. Jumper the spark plug end of the companion cylinder ignition wire to engine ground. The companion cylinder is the cylinder that shares the same ignition coil (i.e., 1/4, 2/5, 3/6).
  3. Crank the engine while observing the J 26792 .

Is spark present?

--

Go to Step 15

Go to Step 11

11
  1. Measure the resistance of the ignition wires associated with the cylinders that were indicated as misfiring.
  2. Replace the ignition wires if the resistance measures more than the specified value.

Did you find and correct the condition?

600 ohms per foot (1 968 ohms per meter)

Go to Step 18

Go to Step 12

12
  1. Measure the resistance of the secondary ignition coils associated with the misfiring cylinders.
  2. If the resistance is not between the specified values, replace the malfunctioning ignition coils. Refer to Ignition Coil Replacement .

Did you find and correct the condition?

5K-8K ohms (5,000-8,000 ohms)

Go to Step 18

Go to Step 13

13
  1. Remove the ignition coils associated with the misfiring cylinders.
  2. Inspect the coils for signs of carbon tracking, cracks, or other damage. Refer to Ignition Coil Replacement .

Did you find and correct the condition?

--

Go to Step 18

Go to Step 14

14
  1. Leave the ignition coils disconnected.
  2. Connect a test lamp across the ignition module primary circuit terminals.
  3. Remove the fuel injector fuse.
  4. Observe the test lamp while cranking the engine.

    5. Important: :  Be sure to reinstall the fuel injector fuse.

Does the test lamp blink?

--

Go to Step 16

Go to Step 17

15
  1. Remove the spark plugs from the cylinders associated with the misfire. Refer to Spark Plug Replacement .
  2. Inspect the spark plugs. Refer to Spark Plug Inspection .

Did you find and correct the condition?

--

Go to Step 18

Go to Diagnostic Aids

16

Replace the ignition coils associated with the misfiring cylinders. Refer to Ignition Coil Replacement .

Did you complete the replacement?

--

Go to Step 18

--

17

Replace the ignition control module. Refer to Ignition Control Module Replacement .

Did you complete the replacement?

--

Go to Step 18

--

18
  1. Review and record Freeze Frame/Failure Records data.
  2. Clear DTCs.
  3. Start and idle the engine.
  4. Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame/Failure Records data).
  5. Monitor the scan tool Misfire Current Cyl # display for each cylinder.

Is Misfire Current Cyl # display incrementing for any cylinder (indicating a misfire currently occurring)?

--

Go to Symptoms - Engine Mechanical in Engine Mechanical

System OK