Circuit Description
The powertrain control module (PCM) uses information from the ignition control (IC) module and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring the 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 three-way catalytic converter to overheat under certain conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for three-way catalytic converter overheating are present.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0125, P0336, P0341, P0502, P0503, P1106, P1107, P1114, P1115, P1121, P1122, P1336, or P1374 are not set. |
| • | The engine speed is between 550-5,850 RPM. |
| • | The system voltage is between 9-18 volts. |
| • | The engine coolant temperature (ECT) indicates an engine temperature between -6°C (21°F) 120°C (248°F). |
| • | The throttle angle is steady. |
Conditions for Setting the DTC
The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels to exceed mandated standard.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | 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 consecutive 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. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive 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 clears after 40 consecutive 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
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 Hist #1-#6) will still contain a value that represents the level of misfire detected on each cylinder.
A misfire DTC may set if components that affect the crankshaft position (CKP) sensor have recently been replaced, and the crankshaft position (CKP) system variation learn procedure has not been performed. If the diagnostic table does not identify a condition, then perform the Crankshaft Position System Variation Learn . The crankshaft position (CKP) system variation learn procedure should be performed if any of the following conditions are true:
| • | The PCM has been replaced. |
| • | DTC P1336 is set. |
| • | The engine has been replaced. |
| • | The crankshaft has been replaced. |
| • | The crankshaft harmonic balancer has been replaced. |
| • | The crankshaft position sensor has been replaced. |
The scan tool displayed misfire counter values (Misfire Hist. #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, inspect for the following conditions:
| • | The secondary ignition wires--Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure less than 600 ohms per foot. |
| • | A damaged or malfunctioning ignition coil--Check for cracks, for carbon tracking, or for other damage. Also check the coil's secondary resistance. The secondary resistance should be between 5,000-8,000 ohms (5-8K ohms). |
Switch the ignition coils, and re-test the coils. If the misfire follows the coil, replace the ignition coil.
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.
If the misfire is random, check for the following conditions:| • | The system grounds--Ensure all connections are clean and properly tightened. |
| • | The mass air flow sensor. |
| 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 with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the Mass Airflow Sensor Replacement . |
| • | A damaged accessory drive belt or driven accessory--A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC. |
| • | The air induction system--Vacuum leaks that cause intake air to bypass the mass air flow (MAF) sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, for incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the exhaust gas recirculation (EGR) valve, and the intake manifold mounting surfaces. |
| • | The fuel pressure--Perform a fuel system pressure test. A malfunctioning fuel pump, plugged filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis . |
| • | The fuel injectors |
| Refer to Fuel Injector Solenoid Coil Test |
| • | Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | The EGR system--Check for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or cause excessive EGR flow. |
| • | An extended idle |
| 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 set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80°C (176°F), the condition described above is very likely the cause of the DTC to set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the electronic brake control module (EBCM) 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.
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The Misfire Current Cyl # display may normally display a small amount of activity, typically 0-10 counts, but should not steadily increment during an entire 200-revolution test sample period.
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Depending on 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 1/2 as great as the misfiring cylinders. When investigating a misfire, always start with items associated with the cylinders that have the largest number of counts stored in the Misfire History Cyl # counter.
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Steps 5-12 test for conditions that can cause a random cylinder misfire.
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Steps 13-23 test for conditions that can cause a non-random or single cylinder misfire.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |||||||||||
Are any other DTCs set? | -- | Go to Step 3 | ||||||||||||
Is Misfire Current # display increments for any cylinder, indicating a misfire currently occurring? | -- | Go to Step 4 | Go to Diagnostic Aids | |||||||||||
View the Misfire History Cyl # display with a scan tool. Does Misfire History Cyl # display a very large value for more than one cylinder? | -- | Go to Step 5 | Go to Step 13 | |||||||||||
Visually and physically inspect the vacuum hoses for splits, for kinks, and for improper connections. Refer to Emission Hose Routing Diagram . If a condition is found, repair as necessary. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 6 | |||||||||||
6 | Inspect the crankcase ventilation valve for improper installation and for damaged O-rings. If a condition is found, repair as necessary. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 7 | ||||||||||
7 | Inspect the throttle body inlet screen for damage or for the presence of foreign objects that may alter the air flow sample through the MAF sensor. If a condition is found, repair as necessary. Refer to Throttle Body Air Inlet Screen Replacement . Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 8 | ||||||||||
8 | Test fuel pressure. Refer to Fuel System Diagnosis . If a condition is found, repair as necessary. Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 9 | ||||||||||
9 | Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . If a condition was found, repair as necessary. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 10 | ||||||||||
10 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 11 | ||||||||||
11 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 12 | ||||||||||
12 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 13 | ||||||||||
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 14 | |||||||||||
14 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 15 | ||||||||||
15 |
The companion cylinder is the cylinder that shares the same ignition coil (i.e., 1/4; 2/5; 3/6). Is spark present? | -- | Go to Step 20 | Go to Step 16 | ||||||||||
16 |
Important: If carbon tracking or terminal discoloration is apparent at the ignition coil end of any of the ignition wires, replace the affected ignition wire and the associated ignition coil. Refer to Ignition Coil Replacement . Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 17 | ||||||||||
17 | Measure the resistance of the spark plug wires associated with the cylinders that were indicated as misfiring. Refer to Ignition System Specifications . Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 18 | ||||||||||
18 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 19 | ||||||||||
19 |
Did you find and correct the condition? | 5-8K ohms (5,000-8,000 ohms) | Go to Step 25 | Go to Step 24 | ||||||||||
20 |
Did you find and correct the condition? | -- | Go to Symptoms - Engine Mechanical in Engine Mechanical | Go to Step 21 | ||||||||||
21 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 22 | ||||||||||
22 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Step 23 | ||||||||||
23 |
Did you find and correct the condition? | -- | Go to Step 25 | Go to Diagnostic Aids | ||||||||||
24 | Replace the ignition control module. Refer to Ignition Control Module Replacement . Did you complete the replacement? | -- | Go to Step 25 | -- | ||||||||||
25 |
Does the DTC run and pass? | -- | Go to Step 26 | Go to Step 2 | ||||||||||
26 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
