Circuit Description
The powertrain control module (PCM) has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the ignition control module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring. If 2 percent or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 11 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible 3-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.
Conditions for Running the DTC
| • | None of the following DTCs are set: |
| - | Vehicle speed sensor (VSS) |
| - | Throttle position (TP) sensor |
| - | Engine coolant temperature (ECT) sensor |
| - | Crankshaft position (CKP) sensor |
| - | Camshaft position (CMP) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Mass air flow (MAF) sensor |
| • | The engine speed is between 550 and 5800 RPM. |
| • | The system voltage is between 9 and 18 volts. |
| • | The ECT indicates an engine temperature between -6°C (20°F) and 120°C (248°F). |
Conditions for Setting the DTC
The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause 3-way catalytic converter damage or emissions levels to exceed mandated standards.
Action Taken When the DTC Sets
| • | The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed unless three way converter damage is possible in which case the light will illuminate after the first failure. |
| • | If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire History Cyl #1-#6) will still contain a value that represents the level of misfire detected on each cylinder. The scan tool displayed misfire counter values (Misfire History Cyl. #1 through #6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil - 1/4, 2/5, 3/6), or is random. If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:
A misfire DTC may set if components that affect the crankshaft position sensor have been recently replaced, and the Crankshaft Position System Variation Learn has not been performed. If the diagnostic table does not identify a problem then perform the Crankshaft Position System Variation Learn .
If the misfire is random, check for the following conditions:
| • | Crankshaft Position System Variation |
| Refer to Crankshaft Position System Variation Learn . |
| The crankshaft position system variation compensating values are stored in the PCM non-volatile memory after a learn procedure has been performed. If the actual crankshaft position variation does not match the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set. The crankshaft position system variation learn procedure is required when any of the following service procedures have been performed: |
| - | The PCM has been replaced. |
| - | DTC P1336 has set. |
| - | The engine has been replaced. |
| - | The crankshaft has been replaced. |
| - | The harmonic balancer has been replaced. |
| - | The crankshaft position sensor has been replaced. |
| - | Any engine repairs which disturb the crankshaft/harmonic balancer to crankshaft position sensor relationship. |
| • | Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. |
| If any spark plug wire resistance is more than the specified value, replace the affected spark plug wires. |
VIN 1 Spark Plug Wires Resistance
3280-4921 ohms per meter (1000-1500 ohms per
foot)
VIN K Spark Plug Wires Resistance
3280-4921 ohms per meter (1000-1500 ohms per foot)
| • | A damaged or malfunctioning ignition coil. Check for cracks, carbon tracking, or other damage. Also check the coil secondary resistance. The secondary resistance should be within the specified range. |
Ignition Coils Secondary Resistance
5000-8000 ohms.
| • | Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil. |
| • | Check the system grounds. Ensure all the connections are clean and properly tightened. |
| • | A mass air flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the Failure Records conditions. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor. |
| • | If the PCM stops receiving data from the EBCM/EBTCM, DTC P0300 can set due to a loss of rough road data. Check for stored ABS/TCS DTCs, especially DTCs related to a serial data malfunction. Refer to Diagnostic System Check - ABS in Antilock Brake System . |
| • | Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, an incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces. |
| • | Perform a fuel system pressure test. A malfunctioning fuel pump, plugged fuel filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Pressure Test (VIN K) or to Fuel System Pressure Test (VIN 1) |
| • | Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | Check the EGR system for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow. |
| • | Excessive open loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to be set, review the freeze frame and fail records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures less than 80°C (176°F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 gm/s. |
Important:: If the level of misfire was sufficient to cause possible catalyst damage (if the MIL was flashing), ensure that the DTC P0420 test is completed and passed after verifying the misfire repair.
Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
The numbers below refer the step numbers on the diagnostic table.
-
A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the EBCM/EBTCM may cause a misfire DTC to be set. If any of the indicated DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.
-
The Misfire Current Cyl # display may normally display a small amount of activity (0-10 counts) but should not steadily increment during an entire 200 revolution test sample period.
-
Depending upon the cause of the misfire, the Misfire History Cyl # counter will display a very large number for the misfiring cylinders. Values for the non-misfiring cylinders will be less than half as much as the misfiring cylinders. When investigating a misfire, always start with items associated with the cylinder that has the largest number of counts stored in the Misfire History Cyl # counter.
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If the misfiring cylinders are companion cylinders, the condition is most likely linked to the ignition system.
-
Check for faulty terminal connection, grooves, corrosion, pitting, loose fit.
-
Check for the following conditions that may contribute to the engine misfire: the engine oil pressure, a damaged accessory drive belt or pulley, a damaged driven accessory (generator, water pump, drive belt tensioner, etc.), a loose or broken motor mount.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||||
Are any other DTCs set? | -- | |||||||||||||
Is the Misfire Current # display incrementing for any cylinder, indicating a misfire currently occurring? | -- | |||||||||||||
View the Misfire History Cyl # display on the scan tool. Does Misfire History Cyl # display a very large value for more than one cylinder? | -- | |||||||||||||
Are the misfire values displayed on the Misfire History Cyl # related to companion cylinders (i.e. 1/4, 2/5, 3/6)? | -- | |||||||||||||
6 |
Was a problem found? | -- | ||||||||||||
7 | Check the fuel pressure. Is the fuel pressure within the specified range? | 333-376 kPa (48-55 psi) | Go to Fuel System Pressure Test (VIN K) or to Fuel System Pressure Test (VIN 1) | |||||||||||
8 |
Was a problem found? | -- | ||||||||||||
Important: : If carbon tracking is apparent at either end of the spark plug wires, replace the affected ignition wire and the associated ignition coil or spark plug.
Was a problem found? | -- | |||||||||||||
10 |
Is spark present? | -- | ||||||||||||
11 |
Was a problem found? | 3280-4921 ohms per meter (1000-1500 ohms per foot) | ||||||||||||
12 |
Was a problem found? | 5,000-8,000 ohms | ||||||||||||
13 |
Was a problem found? | -- | ||||||||||||
14 |
Important: : Be sure to reinstall the fuel injector fuse. Does the test lamp blink? | -- | ||||||||||||
15 |
Was a problem found? | -- | Go to Base Engine Misfire Diagnosis in Engine Mechanical. | |||||||||||
16 |
Important: : If carbon tracking is apparent on any of the spark plugs, replace the affected spark plugs and the associated ignition wires.
Was a problem found? | -- | ||||||||||||
Was a problem found? | -- | |||||||||||||
18 |
Was a problem found? | -- | ||||||||||||
19 |
Was a problem found? | -- | Go to Diagnostic Aids | |||||||||||
20 | Replace the ignition coils associated with the misfiring cylinders. Refer to Ignition Coil Replacement . Did you complete the repair? | -- | -- | |||||||||||
21 | Replace the ignition control module. Refer to Ignition Control Module Replacement . Did you complete the repair? | -- | -- | |||||||||||
22 |
Is the Misfire Current Cyl # display incrementing for any cylinder indicating a misfire currently occurring? | -- | System OK |
