| Table 1: | DTC P0300 - Engine Misfire Detected |
Circuit Description
The crankshaft position (CKP) sensor sends pulses to the engine control module (ECM) as the reluctor wheel teeth rotate past the sensor. Besides using these pulses in order to synchronize the ignition and fuel injector operation, the ECM also times the interval between each pulse and compares each new time interval with the previous one in order to determine when an excessive change in the crankshaft speed has occurred. A misfire causes an unexpected change in the crankshaft speed. A certain amount of acceleration and deceleration is expected between each firing stroke, but if the crankshaft speed changes are more than an expected amount, the ECM will interpret this as a misfire. The ECM continuously calculates the crankshaft position from the CKP sensor signals. This information is used in order to determine which cylinder is misfiring so that the ECM can increment the appropriate misfire counter and set the appropriate diagnostic trouble code (DTC), if necessary. DTCs P0301 through P0306 correspond to cylinders 1 through 6. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will set. If the misfire is random and on multiple cylinders, DTC P0300 will set.
Conditions for Running the DTC
| • | One or more of the following DTCs are not set: P0100, P0115, P0116, P0120, P0121, P0335, P0336 |
| • | The engine speed is between 520 RPM and 6,400 RPM. |
| • | The load signal meets the following criteria: |
Engine Speed (RPM) | Load Signal (ms) |
|---|---|
1000 | 1.0 |
2000 | 1.1 |
3000 | 1.1 |
6400 | 2.7 |
| • | The load signal change is less than 1.5 milliseconds per cylinder event. |
| • | The Torque Management is not active. |
| • | The ABS/TC is not active. |
| • | Rough Road is not detected. |
| • | The fuel cutoff is not active (including the traction control, the deceleration, the high speed/RPM ). |
| • | The engine has been running for longer than 5 seconds. |
| • | The IAT is greater than -8°C (17°F). |
| • | The EVAP diagnosis is not active. |
Conditions for Setting the DTC
| • | A misfire that is more than the legislated limit, an amount of misfire ranging from 1.0-12.5 percent. |
| • | The above conditions met for at least 5 camshaft revolutions. |
Action Taken When the DTC Sets
| • | The ECM will illuminate the malfunction indicator lamp (MIL) under the following conditions: |
| - | The ECM determines that the percentage of misfire is high enough in order to cause catalyst damage. The ECM will flash the MIL during the time that the catalyst damaging misfire is present. The ECM will switch OFF the injector, a maximum of 2, related to the misfiring cylinder, and the MIL will illuminate continuously. |
| - | The ECM determines that the percentage of misfire is high enough in order to cause excessive tail pipe emissions. The ECM illuminates the MIL on the next consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions, the load, the RPM, the temperature, as the MIL illuminated in the previous ignition cycle. |
| • | The first drive trip that this diagnostic fails, shows a reading of Fail Pending, this information will store in the Freeze Frame. Any data that was previously stored in the Freeze Frame will be overwritten by the Freeze Frame data for this DTC. The only exception to this is if a Fuel Trim DTC was already recorded in the Freeze Frame. In this case, the fuel trim data stays in the Freeze Frame and the Misfire data will not store. |
Conditions for Clearing the MIL/DTC
| Important: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than three drive trips. Review the Freeze Frame for the last failure conditions. |
| Important: If the fuel cutoff was active as a result of catalyst damaging misfire, and a catalyst damaging misfire no longer exists, the fuel cutoff can only be deactivated by a new ignition cycle. |
| • | The ECM turns OFF the MIL after three consecutive drive trips that the diagnostic runs and does not fail within the same conditions that the DTC last failed. |
| • | A History DTC clears after forty consecutive warm-up cycles if no failures report by this diagnostic or any other emission related diagnostic. |
| • | The ECM battery voltage is interrupted. |
| • | Cleared by a scan tool |
Diagnostic Aids
If P1460 was also set, and no misfire is currently present, the vehicle may simply have been run low on fuel. This DTC being present with a properly functioning fuel sender circuit eliminates the need for a lengthy misfire diagnosis.
A Misfire DTC could be caused by an excessive vibration from sources other than the engine. Check for the following possible sources:
| • | Tire/wheel out of round/balance |
| • | Variable thickness brake rotor/drum |
| • | Drive shaft not balanced |
| • | Certain rough road conditions |
Test Description
The number(s) below refer to the step numbers on the diagnostic table:
Step | Action | Value(s) | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System check? | -- | ||||||||||||||
2 | Are any DTCs from P0201 thru P0206 also set? | -- | Go to the applicable DTC | |||||||||||||
3 | Is DTC P1460 also set? | -- | ||||||||||||||
4 | Are any DTCs from P0301 thru P0306 also set? | -- | Go to the applicable DTC | |||||||||||||
5 | Monitor the Fuel Level Volts with the scan tool. Are the fuel level volts above the specified value? | 6.8 V | ||||||||||||||
6 | Add 5 gallons of fuel to the tank. Do the fuel level volts drop below the specified value? | 6.8 V | ||||||||||||||
7 | Repair the fuel gauge sender/circuit problem. Refer to Electrical Diagnosis, Fuel Gauge. Is the action complete? | -- | -- | |||||||||||||
Are any of the Misfire Current counters incrementing? | -- | |||||||||||||||
9 | Visually/physically inspect the following items:
Was a problem found and corrected? | -- | ||||||||||||||
10 |
Is this DTC set? | -- | Refer to Diagnostic Aids | |||||||||||||
11 | Use a DVM (J 39200 ) in order to measure the resistance of the ignition wire(s) that are indicating a misfire. Then, replace any ignition wires which measure greater than the specified value. Was a problem found and corrected? | cyl 1, 3, and 5: 7-9K ohms cyl 2, 4, and 6: 2-6K ohms | ||||||||||||||
12 | Check the spark plugs for signs of oil or coolant fouling. Was a problem found? | -- | Refer to Engine Mechanical | |||||||||||||
13 | Check the spark plugs for the following conditions:
Was a problem found and corrected? | -- | Refer to Diagnostic Aids | |||||||||||||
14 |
Is this DTC set? | -- | ||||||||||||||
15 | Were there any additional DTCs noted during the Powertrain OBD System Check that have not been diagnosed? | -- | Go to the applicable DTC table | The System is OK |
