Refer to
Ignition System
and
Cell 20: Fuel Injectors
schematics.
Circuit Description
The powertrain control module (PCM) uses the crankshaft position (CKP) sensor and camshaft position (CMP) sensor to determine engine misfire. The CKP sensor and the CMP sensor monitor their respective components and evaluate changes in crankshaft rotational speed for each cylinder. Irregular changes in the crankshaft rotational speed indicate a possible misfire. 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. Each cylinder is monitored individually for a misfire condition. A DTC P0300 indicates that engine misfire was indicated in more than one cylinder at the same time.
Conditions for Running the DTC
| • | Engine speed is between 200 and 4,000 RPM. |
| • | Battery voltage is between 9 volts and 17 volts. |
| • | The engine coolant temperature is between -7°C (20°F) and 123°C (254°F). |
Conditions for Setting the DTC
Misfire is indicated in more than one cylinder simultaneously.
Action Taken When the DTC Sets
| • | The PCM flashes the malfunction indicator lamp (MIL) the first time catalytic converter damage can occur. |
| • | The PCM illuminates the MIL the second time emission thresholds are exceeded (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. The PCM also freezes two scan tool data list parameters related to the misfire condition. The Engine Speed at Misfire and the Load at Misfire scan tool data parameters are fixed at the time the MIL illuminates. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present. |
| • | A DTC will clear after 40 consecutive warm-up cycles have occurred without a fault. |
| • | A DTC can be cleared by using the scan tool Clear Information function. |
Diagnostic Aids
Check for the following conditions:
| • | If any DTCs other than misfire (P0300 to P0304) are present, diagnose those DTCs first. |
| • | If DTCs P0301, P0302, P0303, or P0304 are also present with a DTC P0300, diagnose those DTCs first. |
| • | An intermittent ignition system malfunction (spark plugs, ignition wires, ignition coil) may cause a DTC P0300 to set. Check ignition system performance with an engine oscilloscope. |
| • | The Engine Speed at Misfire and the Load at Misfire scan tool data parameters are fixed at the time the MIL illuminates. These two scan tool displays can be used like two additional lines of Freeze Frame data. Review the Engine Speed at Misfire and the Load at Misfire scan tool parameters located on the scan tool Engine Data list. The information can help determine the vehicle conditions when the misfire occurred. |
| • | Review the misfire counters located in the Engine Data list of the scan tool while the engine is running. If the Total Misfire Current Count data parameter is being populated (count increasing), the misfire condition is present. A current DTC P0300 should indicate misfire activity in the Total Misfire Current Count parameter. Use this information in order to determine if the fault is present now, or is an intermittent malfunction. |
| • | An intermittent can also be the result of a defective CKP sensor signal rotor. Remove the CKP sensor and inspect the signal rotor through the sensor hole. Check the condition of the signal rotor for nicks, dents, missing teeth and foreign material. |
| • | Check for engine overheating. |
| • | Check for a PCV system malfunction. Perform a functional check of the PCV valve. Refer to Crankcase Ventilation System Inspection . |
An intermittent malfunction may be caused by a fault in the ignition system or fuel system electrical circuits. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connectors locks. |
| • | Improperly formed or damaged terminals |
| • | Faulty terminal to wire connections. |
| • | Physical damage to the wiring harness. |
| • | A broken wire inside the insulation. |
| • | Corrosion of electrical connections, splices, or terminals. |
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
-
The OBD System Check 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.
-
This step verifies whether the misfire is present. The scan tool will display increasing counts in the Total Misfire Current Count parameter if a misfire is occurring.
-
This step checks whether the DTC P0300 is the result of a hard failure or an intermittent condition. Operating the vehicle in check mode enhances the PCM's diagnostic capabilities. For additional information on check mode operation refer to the Powertrain Control Module Diagnosis section. The scan tool will display increasing counts in the Total Misfire Current Count parameter if a misfire is occurring.
-
This step is to verify if the misfire is cause by a fault in the ignition system.
-
This step checks for a faulty signal rotor. A faulty signal from the CKP sensor can cause the ignition system to misfire. Visually inspect the signal rotor through the CKP sensor aperture for damaged teeth, missing teeth, foreign material, and mis-alignment.
-
Contaminants in the fuel, such as alcohol or water, can create a misfire condition.
-
A vacuum leak can cause a lean misfire condition.
Step | Action | Value(s) | Yes | No | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||||||||||||||
Did the scan tool indicate any misfire counts? | -- | |||||||||||||||||||
Did a DTC P0300 set or did the scan tool indicate any misfire counts? | -- | Fault not Present-Go to Diagnostic Aids | ||||||||||||||||||
Is spark present at each ignition wire? | -- | |||||||||||||||||||
5 |
Did any of the ignition wires fail any of the above inspections? | 25,000 ohms (25k ohms) | ||||||||||||||||||
6 |
Was a repair necessary? | -- | ||||||||||||||||||
Was a repair necessary? | -- | |||||||||||||||||||
8 |
Did any of the spark plugs need replacement? | -- | ||||||||||||||||||
9 |
Did the fuel injector test lamp flash for each injector while cranking the engine? | -- | ||||||||||||||||||
10 | Perform the Fuel Injector Coil Test and the Fuel Injector Balance Test procedures. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) and Fuel Injector Balance Test . Did any fuel injectors require replacement? | -- | ||||||||||||||||||
11 |
Is the fuel pressure within the specified value? | 301-347 kPa (44-50 psi) | Go to Fuel System Diagnosis | |||||||||||||||||
Was a repair necessary? | -- | |||||||||||||||||||
Was a repair necessary? | -- | |||||||||||||||||||
14 | Replace the faulty ignition coil. Refer to Ignition Coil Replacement . Is the action complete? | -- | -- | |||||||||||||||||
15 | Replace any spark plug wires that were found to be faulty. Is the action complete? | -- | -- | |||||||||||||||||
16 |
Was a basic engine mechanical concern found and repaired? | -- | Go to Diagnostic Aids | |||||||||||||||||
17 |
Did a DTC set or did the scan tool indicate any misfire counts? | -- | Go to the Applicable DTC Table | System OK |
