System Description
The powertrain control module (PCM) uses information from the ignition control module (ICM) 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 supports the following DTC:
DTC P0300 Engine Misfire Detected
Conditions for Running the DTC
| • | DTC P0101, P0102, P0103, P0107, P0108, P0116, P0117, P0118, P0121, P0122, P0123, P0125, if applicable, P0128, P0315, P0335, P0336, P0340, P0341, P0350, P0502, P0503, P1350 are not set. |
| • | The engine speed is between 475-5,850 RPM. |
| • | The ignition voltage is between 9-18 volts. |
| • | The engine coolant temperature (ECT) is between -7 and +125°C (+21 and +257°F). |
| • | The fuel level is more than 10 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 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 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. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:
| • | The control module detects the same misfire failure during 2 consecutive trips. |
| • | The control module detects any misfire failure during any subsequent trip if the conditions at the time of failure meet the following criteria: |
| - | The engine load is within 20 percent of the previous test that failed. |
| - | The engine speed is within 375 RPM of the previous test that failed. |
| - | The engine coolant temperature is in the same range of the previous test that failed. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) at the beginning of the fourth ignition cycle, after 3 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 and related Freeze Frame data clears after 40 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 . |
| - | Worn or damaged accessory drive belt--Refer to Symptoms - 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.
Step | Action | Values | Yes | No | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||||||||
Are any of the Misfire Current counters incrementing? | -- | Go to Step 3 | Go to Diagnostic Aids | |||||||||||||||
3 | Are any other DTCs set? | -- | Go to Step 4 | |||||||||||||||
4 | Can any abnormal engine noise be heard? | -- | 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:
Did you find and correct the condition? | -- | Go to Step 20 | Go to Step 8 | ||||||||||||||
8 |
Does the spark jump the tester gap, and is the spark consistent? | -- | Go to Step 10 | Go to Step 9 | ||||||||||||||
9 |
Is the spark plug wire resistance less than or equal to the specified resistance? | -- | Go to Step 19 | |||||||||||||||
10 |
Does the spark plug appear to be OK? | -- | Go to Step 11 | Go to Step 12 | ||||||||||||||
11 |
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 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 Diagnosis . Did you find and correct the condition? | -- | Go to Step 20 | |||||||||||||||
16 | Perform the fuel system diagnosis. Refer to Fuel System Diagnosis . Did you find and correct the condition? | -- | Go to Step 20 | |||||||||||||||
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 |
Does the DTC run and pass? | 2,500 RPM | Go to Step 22 | Go to DTC P0420 | ||||||||||||||
22 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 23 | ||||||||||||||
23 | In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the Conditions for Running and the Conditions for Setting DTC P0420, and verify that DTC P0420 runs and passes. Refer to DTC P0420 . Does the DTC run and pass? | -- | Go to Step 24 | Go to DTC P0420 | ||||||||||||||
24 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK |
