System 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 occurs. By monitoring the variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A high misfire rate can cause the 3-way catalytic (TWC) converter to overheat under certain conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present.
Conditions for Running the DTC
| • | The engine speed is between 469-5,906 RPM |
| • | The battery voltage is between 9-17 volts |
| • | The engine coolant temperature (ECT) is between -7 to +123°C (20-254°F) |
| • | The engine has been running more than 5 seconds |
| • | The fuel level is above 10 percent |
Conditions for Setting the DTC
The PCM detects a crankshaft rotation speed variation indicating a misfire rate sufficient to cause the emissions levels to exceed a 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
An intermittent DTC can also be the result of a defective reluctor wheel. Remove the crankshaft sensor and inspect the reluctor wheel through the crankshaft sensor mount hole. Inspect the porosity and the condition of the reluctor wheel.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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Whenever DTC P0108 is present, verify that no misfire is present by viewing the misfire counters. DTC P0108 may set as a result of a misfire.
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Inspect for the following conditions:
| • | The wiring for proper connections, pinches, and cuts |
| • | Clean and tight PCM grounds |
| • | The vacuum hoses for splits, kinks, and proper connections as shown on the vehicle emission information label. Inspect thoroughly for any type of leak or restriction. |
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This step determines if the Misfire DTC is the result of a hard malfunction or an intermittent condition.
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If the misfire is not present, operate the vehicle under the conditions found in the Freeze Frame data in order to detect the misfire. Depending on the engine load, the conditions may have to be maintained for up to 20 seconds. If the misfire accumulators start to increment, the misfire is present. A history misfire counter will record the number of misfires that occur until the DTC is cleared. The current counter must count a total of at least 195 misfires before the history counter begins storing the misfire information.
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No spark on one coil may be caused by an open secondary circuit. Therefore, the coils secondary resistance should be checked.
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Use the injector test lamp to test the harness wiring and to test for PCM control of the injectors.
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Perform the Fuel Inj Coil Test on the misfiring cylinder only.
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This step determines if the injector driver circuit is shorted to ground or voltage, or if the PCM is malfunctioning.
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Replace any spark plugs that are worn, cracked, or fouled.
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Since the voltage is supplied to the injector on a single circuit before the injector harness, the malfunction may only be a poor connection or an open in the injector harness. An open before the harness may result in a Cranks But Does Not Run complaint.
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A basic engine problem that affects only a single cylinder, such as valve train, compression, etc., is possible at this point.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
Are any component DTCs set? | -- | Go to Step 3 | ||
Were any repairs necessary? | -- | Go to Step 6 | Go to Step 4 | |
With the engine idling, install a scan tool. Is the misfire current counter incrementing on the cylinder that had a misfire DTC? | -- | Go to Step 5 | Go to Step 6 | |
5 |
Does the spark tester spark? | -- | Go to Step 8 | Go to Step 9 |
Is the misfire current counter incrementing on the cylinder that indicated a misfire? | -- | Go to Step 5 | Go to Step 7 | |
7 | Was a repair made from the visual and physical inspection? | -- | Go to Step 30 | Go to Diagnostic Aids |
8 |
Is the misfire current counter incrementing on the cylinder that indicated a misfire? | -- | Go to Step 13 | Go to Step 22 |
9 |
Is the spark plug cable resistance less than the specified value? | 1,000 ohms/in | Go to Step 10 | Go to Step 23 |
Is the resistance within the specified range? | 5-8 K ohms | Go to Step 11 | Go to Step 24 | |
11 |
Did spark occur on the spark tester? | -- | Go to Step 24 | Go to Step 12 |
12 |
Were any repairs necessary? | -- | Go to Step 30 | Go to Step 25 |
Does the fuel injector test lamp blink? | -- | Go to Step 14 | Go to Step 15 | |
Perform the Injector Coil/Balance Test Procedure . Is the fuel injector OK for the cylinder that had indicated a misfire? | -- | Go to Step 28 | Go to Step 29 | |
15 |
Did the test lamp illuminate? | -- | Go to Step 16 | Go to Step 27 |
Did the test lamp remain OFF? | -- | Go to Step 17 | Go to Step 26 | |
17 |
Did the test lamp remain OFF? | -- | Go to Step 18 | Go to Step 19 |
18 | Inspect the fuel injector harness connector and the PCM harness connectors and connections for an open circuit. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Are the connections OK? | -- | Go to Step 20 | Go to Step 21 |
19 |
Did you find and correct the condition? | -- | Go to Step 30 | Go to Step 20 |
20 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 30 | -- |
21 | Repair the connector or connections. Refer to Connector Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 30 | -- |
Replace the malfunctioning spark plug. Refer to Spark Plug Replacement Did you complete the replacement? | -- | Go to Step 30 | -- | |
23 | Replace the spark plug cable. refer to Spark Plug Wire Replacement . Did you complete the replacement? | -- | Go to Step 30 | -- |
24 | Replace the defective ignition coil. Refer to Ignition Coil Replacement . Did you complete the replacement? | -- | Go to Step 30 | -- |
25 | Replace the ICM. Refer to Ignition Control Module Replacement . Did you complete the repair? | -- | Go to Step 30 | -- |
26 |
Did you complete the repair? | -- | Go to Step 30 | Go to Step 20 |
Repair the open ignition positive voltage circuit for the fuel injector. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 30 | -- | |
Inspect for a basic engine problem, refer to Symptoms - Engine Mechanical in Engine Mechanical. Did you complete the repair? | -- | Go to Step 30 | -- | |
29 | Replace the malfunctioning fuel injector. Refer to Fuel Injector Replacement . Did you complete the repair? | -- | Go to Step 30 | -- |
30 |
Does the DTC run and pass? | -- | Go to Step 31 | Go to Step 2 |
31 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
