GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 Montana Ext. 🢒 DTC 🢒 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 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 3-way catalytic converter (TWC) to overheat under certain driving conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present. If the PCM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 will set.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0107, P0108, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0315, P0336, P0341, P0502, P0503, P1106, P1107, P1114, P1115, P1121, P1122, P1351, P1352, P1361, P1362, P1374 are not set.
    • The engine speed is between 525-5,900 RPM.
    • The ignition voltage is between 9-18 volts.
    • The engine coolant temperature (ECT) is between -7 and +125°C (+21 and +255°F).
    • The fuel level is more than 10 percent.
    • The throttle position (TP) sensor angle is steady within 1 percent.
    • The antilock brake system (ABS) and the traction control system are not active.
    • The transmission is not changing gears.
    • The A/C clutch is not changing states.
    • The PCM is not in fuel shut-off or decel fuel cut-off mode.
    • The PCM is not receiving a rough road signal.

Conditions for Setting the DTC

The PCM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission levels to exceed mandated standards.

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
    • Excessive vibration from sources other than the engine could cause DTC P0300 to set. The following are possible sources of vibration:
       - Variable thickness brake rotors--Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes.
       - Worn or damaged accessory drive belt--Refer to Symptoms - Engine Mechanical in Engine Mechanical.
    • Spray water on the secondary ignition components using a spray bottle. Look and listen for arcing or misfiring.

Test Description

The number below refers to the step number on the diagnostic table.

  1. If the actual CKP variation values are not within the learned values, the misfire counters may increment.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: You must perform the crankshaft position (CKP) system variation learn procedure before proceeding with this diagnostic table. Refer to Crankshaft Position System Variation Learn .

  1. Start the engine.
  2. Allow the engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records.
  3. Monitor all of the Misfire Current counters with the scan tool.

Are any of the Misfire Current counters incrementing?

--

Go to Step 3

Go to Diagnostic Aids

3

Are any DTCs other than P0300 or P0136 set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 4

4

Can any abnormal engine noise be heard?

--

Go to Symptoms - Engine Mechanical in Engine Mechanical

Go to Step 5

5

Observe the Misfire History Cyl# parameter with a scan tool.

Does more than one cylinder indicate a large count value for the Misfire History Cyl# parameter?

--

Go to Step 6

Go to Step 8

6

Are the Misfire History Cyl# counts indicated for companion cylinders 1/4 or 2/5 or 3/6?

--

Go to Step 8

Go to Step 7

7

Inspect the following components:

    • The vacuum hoses and seals for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram .
    • The throttle body and intake manifold for vacuum leaks
    • The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
    • The exhaust gas recirculation (EGR) system components for vacuum leaks or excessive flow-- Refer to Exhaust Gas Recirculation (EGR) System Cleaning .
    • The powertrain control module (PCM) grounds for corrosion and loose connections--Refer to Ground Distribution Schematics in Wiring Systems.
    • The exhaust for restrictions--Refer to Restricted Exhaust in Engine Exhaust.
    • The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 20

Go to Step 8

8
  1. Turn OFF the ignition.
  2. Disconnect the spark plug wire from the spark plug that corresponds to the Misfire Current counters that were incrementing. Refer to Spark Plug Wire Replacement .
  3. Install the J 26792 Spark Tester to a good ground.
  4. 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.
  5. Start the engine.

Does the spark jump the tester gap, and is the spark consistent?

--

Go to Step 10

Go to Step 9

9
  1. Remove the spark plug wire for the affected cylinder. Refer to Spark Plug Wire Replacement .
  2. Inspect the spark plug wire. Refer to Spark Plug Wire Inspection .
  3. Measure the resistance of the spark plug wire with a DMM.

Is the spark plug wire resistance less than the specified resistance?

700 ohms

Go to Electronic Ignition (EI) System Diagnosis

Go to Step 19

10
  1. Remove the spark plug from the cylinder that indicated a misfire.
  2. Inspect the spark plug. Refer to Spark Plug Inspection .

Does the spark plug appear to be OK?

--

Go to Step 11

Go to Step 12

11
  1. Exchange the suspected spark plug with another cylinder that is operating properly. Refer to Spark Plug Replacement .
  2. Operate the vehicle under the same conditions that the misfire occurred.

Did the misfire move with the spark plug?

--

Go to Step 18

Go to Step 15

12

Is the spark plug oil or coolant fouled?

--

Go to Symptoms - Engine Mechanical in Engine Mechanical

Go to Step 13

13

Is the spark plug gas fouled?

--

Go to Step 16

Go to Step 14

14

Does the spark plug show any signs of being cracked, worn, or improperly gapped?

--

Go to Step 17

Go to Step 15

15

Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct the condition?

--

Go to Step 20

Go to Symptoms - Engine Mechanical in Engine Mechanical

16

Perform the fuel system diagnosis. Refer to Fuel System Diagnosis .

Did you find and correct the condition?

--

Go to Step 20

Go to Symptoms - Engine Mechanical in Engine Mechanical

17

Replace or gap the spark plug. Refer to Spark Plug Replacement .

Did you complete the replacement?

--

Go to Step 20

--

18

Replace the faulty spark plug. Refer to Spark Plug Replacement .

Did you complete the replacement?

--

Go to Step 20

--

19

Replace the faulty spark plug wires. Refer to Spark Plug Wire Replacement .

Did you complete the replacement?

--

Go to Step 20

--

20

Was the customer concern the malfunction indicator lamp (MIL) flashing?

--

Go to Step 21

Go to Step 22

21
  1. Operate the vehicle at the specified value for 4 minutes.
  2. Operate the vehicle within the Conditions for Running the DTC P0420 as specified in the supporting text. Refer to DTC P0420 .

Does the DTC run and pass?

2,500 RPM

Go to Step 22

Go to DTC P0420

22
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 23

23

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK