P0300

CIRCUIT DESCRIPTION
The DTC P0300 Engine Misfire Detected diagnostic illuminates the malfunction indicator lamp (MIL) when a noncylinder specific misfire is present. 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 the crankshaft rotational speed for each cylinder. Irregular changes in the crankshaft rotational speed indicate a possible misfire. The 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 400-4,000 RPM.
- Battery voltage is between 9-17 volts.
- Engine coolant temperature (ECT) is between -7 to +123°C (20-254°F).

CONDITIONS FOR SETTING THE DTC
A 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 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 the engine speed at Misfire and the Load at Misfire scan tool data parameters at the time the MIL illuminates.

CONDITIONS FOR CLEARING THE MIL/DTC
- The PCM turns OFF the MIL after three consecutive trips without a fault.
- The PCM clears a History DTC after 40 consecutive warm-up cycles without a fault.
- You use the scan tool Clear DTC 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 DTC P0300, diagnose those DTCs first.
- An intermittent ignition system malfunction may cause DTC P0300 to set. Check the 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 2 scan tool displays can be used like 2 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 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 or 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.
- A mass air flow (MAF) sensor output that is incorrect may cause the PCM to command the fuel system to go lean. Check the performance of the MAF sensor. If the HO2S voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition that can cause engine misfire.
- Check for engine overheating.
- Check for a positive crankcase ventilation (PCV) system malfunction. Perform a functional check of the PCV valve.

An intermittent malfunction may be caused by a fault in the ignition system or fuel system electrical circuits. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions. Intermittent Conditions

Repair any electrical circuit faults that were found.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

TEST DESCRIPTION

Step 1:

Steps 2-4:

Steps 5-6:

Steps 7-9:

Steps 10-12:

Steps 13-15:

Steps 16-17:

The numbers below refer to the step numbers in the diagnostic table.
1. The Diagnostic System Check-Computers and Controls Systems 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.
2. 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.
3. 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 diagnostic capabilities of the PCM. For additional information on Check mode operation, refer to the Powertrain Control Module Diagnosis. The scan tool will display increasing counts in the Total Misfire Current Count parameter if a misfire is occurring.
4. Fault not present indicates that the condition that caused the DTC P0300 to set is intermittent and not currently present. If no other DTCs are stored, refer to Diagnostic Aids for additional information on diagnosing an intermittent DTC P0300.
12. Contaminants in the fuel, such as alcohol or water, can create a misfire condition.
13. A vacuum leak can cause a lean misfire condition.