For 1990-2009 cars

### Circuit Description

The DTC P0300 Engine Misfire Detected diagnostic illuminates the malfunction indicator lamp (MIL) when a noncylinder specific misfire is present. The powertrain control module (PCM) uses the crankshaft position (CKP) sensor and camshaft position (CMP) sensor to determine engine misfire. The CKP sensor and the CMP sensor monitor their respective components and evaluate changes in the crankshaft rotational speed for each cylinder. Irregular changes in the crankshaft rotational speed indicate a possible misfire. The MIL illuminates when the misfire rate equals or exceeds a pre-determined count. A misfire rate that is high enough can cause the catalytic converter to overheat under certain driving conditions. The MIL will flash ON and OFF when the conditions for catalytic converter overheating are present. Each cylinder is monitored individually for a misfire condition. A DTC P0300 indicates that engine misfire was indicated in more than one cylinder at the same time.

### Conditions for Running the DTC

• | The engine speed is between 200-4,000 RPM.

• | The battery voltage is between 9-17 volts.

• | The engine coolant temperature (ECT) is between -7 to +123°C (20-254°F).

### Conditions for Setting the DTC

A misfire is indicated in more than one cylinder simultaneously.

### Action Taken When the DTC Sets

• | The PCM flashes the malfunction indicator lamp (MIL) the first time catalytic converter damage can occur.

• | The PCM illuminates the MIL the second time the diagnostic fails.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. The PCM also freezes the engine speed at Misfire and the Load at Misfire scan tool data parameters at the time the MIL illuminates.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.

• | A DTC will clear after 40 consecutive warm-up cycles have occurred without a fault.

• | A DTC can be cleared by using the scan tool Clear Information function.

### Diagnostic Aids

Check for the following conditions:

• | If any DTCs other than misfire (P0300 to P0304) are present, diagnose those DTCs first.

• | If DTCs P0301, P0302, P0303, or P0304 are also present with DTC P0300, diagnose those DTCs first.

• | An intermittent ignition system malfunction may cause DTC P0300 to set. Check the ignition system performance with an engine oscilloscope.

• | The engine speed at Misfire and the Load at Misfire scan tool data parameters are fixed at the time the MIL illuminates. These 2 scan tool displays can be used like 2 additional lines of Freeze Frame data. Review the engine speed at Misfire and the Load at Misfire scan tool parameters located on the scan tool Engine Data list. The information can help determine the vehicle conditions when the misfire occurred.

• | Review the misfire counters located in the Engine Data list of the scan tool while the engine is running. If the Total Misfire Current Count data parameter is increasing, the misfire condition is present. A current DTC P0300 should indicate misfire activity in the Total Misfire Current Count parameter. Use this information in order to determine if the fault is present or an intermittent malfunction.

• | An intermittent can also be the result of a defective CKP sensor signal rotor. Remove the CKP sensor and inspect the signal rotor through the sensor hole. Check the condition of the signal rotor for nicks, dents, missing teeth, and foreign material.

• | A mass air flow (MAF) sensor output that is incorrect may cause the PCM to command the fuel system to go lean. Check the performance of the MAF sensor. If the HO2S voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition that can cause engine misfire.

• | Check for engine overheating.

• | Check for a positive crankcase ventilation (PCV) system malfunction. Perform a functional check of the PCV valve.

An intermittent malfunction may be caused by a fault in the ignition system or fuel system electrical circuits. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

### Test Description

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

1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step verifies whether the misfire is present. The scan tool will display increasing counts in the Total Misfire Current Count parameter if a misfire is occurring.

3. This step checks whether the DTC P0300 is the result of a hard failure or an intermittent condition. Operating the vehicle in Check mode enhances the diagnostic capabilities of the PCM. For additional information on Check mode operation, refer to the Powertrain Control Module Diagnosis. The scan tool will display increasing counts in the Total Misfire Current Count parameter if a misfire is occurring.

4. Fault not present indicates that the condition that caused the DTC P0300 to set is intermittent and not currently present. If no other DTCs are stored, refer to Diagnostic Aids for additional information on diagnosing an intermittent DTC P0300.

5. Contaminants in the fuel, such as alcohol or water, can create a misfire condition.

6. A vacuum leak can cause a lean misfire condition.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Install a scan tool.
2. Start the engine.
3. Monitor the Total Misfire Current Count parameter on the scan tool.

Did the scan tool indicate any misfire counts?

--

Go to Step 5

Go to Step 3

3 |

1. Perform the scan tool Clear DTC Information function.
2. Place the vehicle into Check mode, using the scan tool.
3. Operate the vehicle within the Freeze Frame conditions as specified.
4. Monitor the Total Misfire Current Count parameter on the scan tool.

Did DTC P0300 set or did the scan tool indicate any misfire counts?

--

Go to Step 5

Go to Step 4

4 | The fault is not present. Are there any DTCs stored that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Diagnostic Aids
5 |

1. Disconnect all the fuel injector electrical connectors.
2. Remove the ignition coil 1 . Refer to Ignition Coil Replacement .
3. Connect the ignition coil electrical connector.
4. Connect a J 26792 Spark Tester or an equivalent to the ignition coil 1 boot.
5. Crank the engine and observe the spark tester.
6. A crisp, blue spark should be observed.
7. Install the ignition coil.
8. Repeat Step 2 through Step 5 for each of remaining the ignition coils.

Is a crisp blue spark present for each ignition coil?

--

Go to Step 7

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Exchange the ignition coils. Replace the suspected faulty ignition coil with a coil that produced good spark.
3. Connect a J 26792 or an equivalent to the ignition coil boot.
4. Crank the engine while observing the spark tester.

Was a crisp blue spark present?

--

Go to Step 14

Go to Step 15

7 |

1. Perform a visual and physical inspection of the ignition coil boots and spark plugs for the following conditions:

• | Any water intrusion

• | Any corroded or poor connections to the spark plugs

• | Any carbon tracking

2. Repair the condition or replace the faulty coil as necessary. Refer to Ignition Coil Replacement or Spark Plug Replacement .

Was a repair necessary?

--

Go to Step 17

Go to Step 8

8 |

1. Remove the spark plugs. Refer to Spark Plug Replacement .
2. Visually and physically inspect the spark plugs for any of the following conditions:

• | Any oil fouling

• | Any fuel fouling

• | Any evidence of coolant in the cylinder

• | An incorrect gap

• | A loose center electrode

• | A cracked porcelain

• | Any carbon tracking

• | A loose terminal end

3. Repair or replace as necessary.

Was a faulty spark plug condition found and repaired?

--

Go to Step 17

Go to Step 9

9 |

1. Turn OFF the ignition.
2. Disconnect all 4 fuel injector electrical connectors.
3. Install a J 34730-2B Fuel Injector Test Lamp, or equivalent, into each fuel injector harness connector.
4. Observe the injector test lamp for each fuel injector while cranking the engine.

Does the fuel injector test lamp flash for each injector while cranking the engine?

--

Go to Step 10

Go to Fuel Injector Circuit Diagnosis

10 | Perform the Fuel Injector Coil Test and the Fuel Injector Balance Test procedures. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Fuel Injector Balance Test with Special Tool .

Did any fuel injectors require replacement?

--

Go to Step 17

Go to Step 11

11 |

1. Turn OFF the ignition.
2. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
3. Install a fuel pressure gauge. Refer to Fuel Pressure Gage Installation and Removal .
4. Install a scan tool.
5. Turn ON the ignition, with the engine OFF.
6. Select Fuel Pump in the Engine Output Controls menu of the scan tool.
7. Turn ON the fuel pump, using the scan tool.
8. Observe the fuel pressure indicated by the fuel pressure gauge with the fuel pump running.

Is the fuel pressure within the specified range?

301-347 kPa (44-50 psi)

Go to Step 12

Go to Fuel System Diagnosis

12 |

1. Check a fuel sample for the following concerns:

• | Any alcohol or other contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | The fuel type and quality

2. Replace the fuel as necessary.

Was a repair necessary?

--

Go to Step 17

Go to Step 13

13 |

1. Visually and physically inspect the following for vacuum leaks:

• | The PCV valve

• | The intake manifold gasket

• | The vacuum lines, the vacuum hoses, and the fittings

2. Repair any vacuum leaks as necessary.

Was a repair necessary?

--

Go to Step 17

Go to Step 16

14 | Replace the faulty ignition coil. Refer to Ignition Coil Replacement . Is the action complete? | -- | Go to Step 17 | --
15 |

1. Inspect the electrical circuits of the faulty ignition coil for the following conditions:

• | An open in the ignition coil ground circuit

• | An open in the positive voltage supply circuit

• | An open in the trigger signal circuit

• | No trigger signal generated by the PCM

2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems or Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 17

--

16 |

1. Check for any of the following engine mechanical concerns:

• | A worn or faulty camshaft

• | Any leaky or sticky valves

• | Any excessive valve deposits

• | Any weak valve springs

• | An incorrect valve timing

• | Any intake manifold runner leaks or obstructions

• | Any broken or worn piston rings

• | A leaking head gasket

2. Repair as necessary. Refer to Engine Compression Test in Engine Mechanical.

Was a basic engine mechanical concern found and repaired?

--

Go to Step 17

Go to Diagnostic Aids

17 |

1. Perform the scan tool Clear DTC Information function.
2. Operate the vehicle within the Freeze Frame conditions that set the DTC.
3. Monitor the Total Misfire Current Count parameter on the scan tool.

Did a DTC set or did the scan tool indicate any misfire counts?

--

Go to Diagnostic Trouble Code (DTC) List

System OK