For 1990-2009 cars

### Circuit Description

The DTC P0300 Engine Misfire Detected diagnostic monitors the engine for misfire. The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring changes in the crankshaft rotation for each cylinder the PCM counts individual misfire events. The malfunction indicator lamp (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.

### Condition for Running the DTC

• | The barometric pressure (BARO) is more than 75 kPa

• | The engine coolant temperature is more than -8°C (+18°F).

• | The intake air temperature is more than -8°C (+18°F).

• | The engine speed is less than 4,000 RPM.

• | The fuel tank level is more than 15 percent.

• | The throttle position (TP) sensor change is less than 1 degree/10 ms.

### Conditions for Setting the DTC

• | The misfire rate at 200 engine revolutions is more than a specified value in 2 or more cylinders.

• | The misfire rate at 1,000 engine revolutions is more than a specified value in 2 or more cylinders.

• | The conditions are present for 5 seconds after engine start to 1 second from fuel shut-off.

### Action Taken When the DTC Sets

• | The PCM flashes the MIL the first time catalytic converter damage occurs.

• | The PCM illuminates the MIL the second time emission thresholds are exceeded or 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.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after 3 consecutively passing trips without a fault present.

• | A history DTC clears after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool Clear DTC Information function in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | If any DTCs other than misfire (P0300 to P0306) are present, diagnose those DTCs first.

• | Engine overheating

| Refer to Engine Overheating in Engine Cooling.

• | Any engine vacuum leaks

• | Inspect for the correct ignition timing and adjust as necessary--Refer to Ignition System Timing Adjustment .

• | Contaminated fuel or inferior quality fuel

| Refer to Alcohol/Contaminants-in-Fuel Diagnosis

• | A misfire DTC can also be the result of a defective crankshaft signal rotor--Remove the crankshaft position (CKP) sensor and inspect the crankshaft signal rotor through the sensor hole.

• | An intermittent ignition system malfunction in the spark plugs, the ignition wires, or the ignition coils--Check the ignition system performance with an engine oscilloscope.

• | An intermittent engine misfire may be caused by ignition system components that are susceptible to moisture. Spray water on the ignition system components and wiring in order to check for an engine miss or crossfire.

Check for other intermittent conditions. Refer to Intermittent Conditions .

If a DTC P0300 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0300 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0300 appears in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has not yet run. When the DTC P0300 does not appear in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0300 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

If a DTC P0300 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set. When the fault cannot be duplicated, wet the secondary ignition system with water and operate the vehicle under the conditions that the DTC 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 checks whether the malfunction that caused the DTC P0300 is still present. Driving the vehicle under these conditions will verify whether the fault is present. After performing the scan tool Clear DTC Information function, the DTC P0300 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If DTC P0300 appears in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has not yet run. When DTC P0300 does not appear in the Not Ran Since Code Cleared list, the DTC P0300 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the DTC P0300 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status in order to determine the outcome of the diagnostic test involved.

3. This step checks the ignition coil for external faults.

4. This step checks for the electrical signal from the PCM necessary for the operation of the fuel injector. If the injector test lamp does not illuminate the fuel injector control circuit is diagnosed for a faulty condition.

5. This step checks for faulty fuel injectors that can cause a misfire condition and a DTC P0300 to set.

6. This step checks for a mechanical fault as the cause of the misfire condition. Low engine compression and improper valve lash can cause a rough idling condition.

7. This step checks for faults in the CMP sensor and circuit that can cause incorrect ignition reference signals. Without continuous accurate reference signals, there is no dependable spark or fuel injector pulse. Erratic, missing, or incorrect engine reference signals can cause the engine to misfire.

8. This step checks for a faulty CMP sensor signal rotor. Visually inspect the teeth of the signal rotor through the CMP sensor housing aperture for damage, foreign material, and mis-alignment.

Step | Action | 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 | Start the engine. Is a misfire present at idle? | Go to Step 4 | Go to Step 3
3 |

1. Install a scan tool.
2. Turn ON the ignition, leaving the engine OFF.
3. Perform the scan tool Clear DTC Information function.
4. Operate the vehicle within the Freeze Frame data as specified or until the DTC P0300 diagnostic test has run. Refer to the Test Description.

Is a DTC P0300 set?

Go to Step 4

Go to Diagnostic Aids

4 |

1. Perform a visual and a physical inspection for any of the following conditions:

• | An improperly operating or leaking positive crankcase ventilation (PCV) valve

• | Refer to Crankcase Ventilation System Inspection in Engine Mechanical.

• | An incorrect PCV valve

• | A vacuum leak at the intake manifold or the gasket

• | A vacuum leak at a cracked, split, or worn vacuum hose

2. Repair as necessary.

Was a repair necessary?

Go to Step 18

Go to Step 5

5 |

1. Disconnect the fuel injector electrical connectors.
2. Remove the #1 ignition coil.
3. Install a J 26792 Spark Tester or an equivalent to the #1 ignition coil spark plug boot.
4. Check for spark while cranking the engine.
5. Repeat Steps 2 through 3 for the other ignition coils.

Did the test of each ignition coil indicate a crisp blue spark while cranking the engine?

Go to Step 9

Go to Step 6

6 |

1. Check the ignition coils and spark plug boots for the following conditions:

• | Any cracks

• | Any corrosion on the terminals

• | Any carbon tracking

• | Any moisture

2. Replace or repair as necessary.

Was a repair necessary?

Go to Step 18

Go to Step 7

7 |

1. Turn OFF the ignition.
2. Swap the ignition coils. Replace the suspected faulty ignition coil with a functioning ignition coil.
3. Install a J 26792 or an equivalent and test for spark, as performed in step 4.

Did the test indicate a crisp blue spark while cranking the engine?

Go to Step 8

Go to Electronic Ignition (EI) System Diagnosis

8 | Replace the faulty ignition coil. Refer to Ignition Coil Replacement . Is the action complete? | Go to Step 18 | --
9 |

1. Remove the spark plugs. Refer to Spark Plug Replacement .
2. Visually and physically inspect each spark plug for correct performance and condition. Refer to Spark Plug Inspection .
3. Replace as necessary. Refer to Spark Plug Replacement .

Did any spark plugs need replacement?

Go to Step 12

Go to Step 10

10 |

1. Turn OFF the ignition.
2. Disconnect the fuel injector electrical connectors.
3. Install a J 34730-2B Injector Test Lamp or an equivalent into each of the fuel injector harness connectors one at time.
4. Observe the injector test lamp for each injector while cranking the engine.

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

Go to Step 11

Go to Fuel Injector Circuit Diagnosis

11 | Check the fuel injectors for proper operation. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and to Fuel Injector Balance Test with Special Tool . Was a faulty fuel injector found and replaced? | Go to Step 18 | Go to Step 14
12 | Was the spark plug replaced because of a rich fueling condition or wet fuel fouling? | Go to Fuel System Diagnosis | Go to Step 13
13 | Was the spark plug replaced because of oil or coolant fouling? | Go to Step 15 | Go to Step 18
14 |

1. Inspect the exhaust gas recirculation (EGR) valve for excessive flow or sticking open. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
2. Repair as necessary.

Was a repair necessary?

Go to Step 18

Go to Step 15

15 |

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 broken or leaky piston rings

• | Any weak valve springs

• | An incorrect valve timing

• | Any intake leaks

• | A leaking head gasket

• | A loose or broken motor mount

2. Repair as necessary. Refer to Diagnostic Starting Point - Engine Mechanical in Engine Mechanical.

Was a basic engine mechanical concern found and repaired?

Go to Step 18

Go to Step 16

16 |

1. Remove the CMP sensor. Refer to Camshaft Position Sensor Replacement .
2. Visually inspect the CMP sensor for the following conditions:

• | Any physical damage

• | Any foreign material on the sensor surface

• | Any water or corrosion at the electrical terminals

3. Repair as necessary.

Was a repair necessary?

Go to Step 18

Go to Step 17

17 |

1. Visually inspect the CMP sensor signal rotor in the CMP sensor housing for the following conditions:

• | Any damaged teeth

• | Any foreign material

• | The correct installation

2. Repair as necessary.

Was a repair necessary?

Go to Step 18

Go to Diagnostic Aids

18 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 19

Go to Step 2

19 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | Go to Diagnostic Trouble Code (DTC) List | System OK