For 1990-2009 cars

### System Description

The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the ignition control (IC) module 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

• | DTC P0105, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0171, P0172, P0336, P0340, P0341, P0502, P0503, P0506, P0507, P0601, P0742, P1133, P1336, P1621 are not set.

• | The engine speed is between 469-6,400 RPM.

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

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

• | The engine has been running for more that 5 seconds.

• | The fuel level is more than 10 percent.

### 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.

### 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 counters with the scan tool.

Are any of the Misfire Current counters incrementing?

--

Go to Step 3

Go to Intermittent Conditions

3 | Are any other DTCs 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 | Go to Step 5
5 | Inspect the following components:

• | The vacuum hoses and seals for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram .

• | The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Functional Test in Engine Mechanical.

• | The PCM grounds for corrosion and loose connections--Refer to Ground Distribution Schematics in Wiring Systems.

• | The exhaust system 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 19

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Remove the fuel pump relay. Refer to Relay Replacement in Wiring Systems.
3. Remove the ignition coil housing assembly, keeping the ignition control (IC) module assembly connected to the harness connector. Refer to Ignition Coil Housing Replacement .

Important: Not grounding the IC module housing may cause an erratic spark.

4. Connect a jumper wire between the top of the IC module and a good ground.
5. Install the J 36012-A Ignition System Diagnosis Harness.
6. Install a J 26792 Spark Tester on the #1 spark plug jumper wire.
7. Ground the #4 spark plug jumper wire. The #4 wire is the companion to #1.
8. Crank the engine with the remaining spark plug wires connected.
9. Repeat the above steps by installing the spark tester on #4 and grounding #1. Do the same for the #2 and the #3 spark plugs. Ensure the companion wire is grounded.

Does the spark tester spark on all cylinders?

--

Go to Step 7

Go to Electronic Ignition (EI) System Diagnosis

7 |

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

Does the spark plug appear to be OK?

--

Go to Step 8

Go to Step 9

8 |

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 15

Go to Step 12

9 | Is the spark plug oil or coolant fouled? | -- | Go to Symptoms - Engine Mechanical in Engine Mechanical | Go to Step 10
10 | Is the spark plug gas fouled? | -- | Go to Step 13 | Go to Step 11
11 | Did the spark plug show any signs of being cracked, worn, or improperly gapped? | -- | Go to Step 14 | Go to Step 12
12 | Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test . Did you find and correct the condition? | -- | Go to Step 19 | Go to Symptoms - Engine Mechanical in Engine Mechanical
13 | Perform the fuel system diagnosis. Refer to Fuel System Diagnosis . Did you find and correct the condition? | -- | Go to Step 16 | Go to Symptoms - Engine Mechanical in Engine Mechanical
14 | Replace or gap the spark plug. Refer to Spark Plug Replacement . Did you complete the action? | -- | Go to Step 16 | --
15 | Replace the faulty spark plug. Refer to Spark Plug Replacement . Did you complete the replacement? | -- | Go to Step 16 | --
16 | Was the customer concern the MIL flashing? | -- | Go to Step 17 | Go to Step 18
17 |

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 18

Go to DTC P0420

18 |

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 19

19 | 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