For 1990-2009 cars

### System Description

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 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.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0300 Engine Misfire Detected

### Conditions for Running the DTC

• | DTCs P0013, P0014, P0016, P0106, P0107, P0108, P0117, P0118, P0120, P0125, P0128, P0218, P0220, P0315, P0336, P0340, P0341, P0365, P0366, P0502, P0503, P1114, P1115, P2125, P2135 are not set.

• | The engine is running.

• | The ignition voltage is between 10-18 volts.

• | The engine coolant temperature (ECT) is between -7 and +130°C (+19 and +266°F).

• | The throttle angle is steady.

• | The A/C compressor clutch is not changing state.

• | The exhaust gas recirculation (EGR) flow test is inactive.

• | The fuel level is above 10 percent.

• | The fuel cut-off or decel fuel cut-off (DFCO) mode is not active.

• | The PCM is not receiving a rough road signal.

• | DTC P0300 runs continuously when the above conditions are met.

### 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:

- | Thickness variation of the brake rotors--Refer to Symptoms - Hydraulic Brakes .

- | The drive shaft not balanced--Refer to Vibration Analysis - Driveline .

- | Worn or damaged accessory drive belt--Refer to Symptoms - Engine Mechanical .

• | There may be more or less cylinders actually misfiring than indicated by the scan tool.

### Test Description

The numbers below refer to the step numbers 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 - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
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 Diagnostic Aids

3 | Are any other DTCs set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | 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 throttle body and intake manifold for vacuum leaks

• | The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Inspection/Description .

• | The powertrain control module (PCM) grounds for corrosion and loose connections--Refer to Ground Distribution Schematics .

• | The exhaust system for restrictions--Refer to Restricted Exhaust .

• | The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 13

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Remove the fuel pump relay. Refer to Relay Replacement .
3. Remove the ignition coil for the affected cylinder, keeping the ignition connected to the harness connector. Refer to Ignition Coil Replacement .

Important: Not grounding the ignition coil may cause an erratic spark.

4. Connect a jumper wire between the ignition coil and a good ground.
5. Install the J 26792 Spark Tester on the ignition coil and a good ground.
6. Crank the engine.

Does the spark tester spark?

--

Go to Step 8

Go to Step 7

7 | Inspect the affected cylinder spark plug boot for a missing or damaged ignition coil spring. Did you find and correct the condition? | -- | Go to Step 15 | Go to Electronic Ignition (EI) System Diagnosis
8 |

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 9

Go to Step 10

9 |

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 13

10 | Is the spark plug oil or coolant fouled? | -- | Go to Symptoms - Engine Mechanical | Go to Step 11
11 | Is the spark plug gas fouled? | -- | Go to Step 13 | Go to Step 12
12 | Did the spark plug show any signs of being cracked, worn, or improperly gapped? | -- | Go to Step 14 | Go to Step 13
13 | Perform the fuel injector coil test. Refer to Fuel Injector Diagnosis . Did you find and correct the condition? | -- | Go to Step 15 | Go to Symptoms - 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 malfunction indicator lamp (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 DTC P0420 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 - Vehicle | System OK