The crankshaft position (CKP) sensor sends pulses to the powertrain control module (PCM) as the reluctor wheel teeth rotate past the CKP sensor. The PCM uses the CKP pulses to synchronize the ignition and fuel injector operation, and to time the interval between each CKP pulse. The PCM determines when an excessive change in crankshaft speed occurs by comparing each new time interval with the previous interval. A misfire causes an unexpected change in the crankshaft speed. A certain amount of acceleration or deceleration is expected between each firing stroke, but if the crankshaft speed changes more than an expected amount, the PCM interprets this as a misfire. The interval between CKP sensor pulses is extremely small. At high engine speeds, slight variations in the following components make misfire detection difficult:

The PCM learns variations during the Crankshaft Position System Variation Learning Procedure. The PCM compensates for these variations when performing misfire calculations. Only a scan tool can command the PCM to perform the Crankshaft Position System Variation Learning Procedure again.

Perform the learning procedure after the following actions:

Important: A PCM power disconnect with the ignition ON may erase the stored pulse value and set the DTC P1336.

Disconnecting the PCM will not erase the learned Crankshaft Position System Variation as long as the ignition switch is in the OFF position.

Important: Reprogramming the PCM does not require running the Crankshaft Position System Variation Learn Procedure unless the PCM is new.

DTC P1336 sets if the Crankshaft Position System Variation is not within an acceptable range, or can not be learned.

There are no crankshaft position (CKP) or camshaft position (CMP) sensor DTCs.

The PCM has not learned the Crankshaft Position System Variation.

Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

If the Crankshaft Position System Variation Learn Procedure cannot be learned, check for the following conditions:

The misfire diagnostic is not operable when the engine speed is above 1,000 RPM and DTC P1336 is set.

When the learn procedure is in progress and the engine speed is at 4,000 RPM, a decrease in engine speed will occur. During the engine coast down, the PCM monitors the CKP sensor signal and stores the pattern. This pattern is used to determine if the crankshaft speed variations are due to misfire.

The numbers below refer to the step numbers on the diagnostic table.

2. The PCM enables the learn process after the engine coolant temperature of 65°C (150°F) is reached. Engine deceleration occurs when the learn procedure is in progress. The PCM monitors and records the CKP sensor signal during engine coast-down conditions.

3. Test in Progress on the scan tool indicates the PCM is performing the learn procedure. The learn procedure will not be allowed if the PCM receives the following information:

•	An incorrect crankshaft to camshaft pulse ratio

•	An incorrect transmission range signal input

•	Corrupted Class 2 communication

•	No change in the brake switch input

4. If a repair is necessary due to a condition which prevented the learn procedure from enabling, the Crankshaft Position System Variation Learning Procedure must be performed.

5. The ignition must be in the OFF position for at least 10 seconds after the learn procedure is complete and the engine RPM has returned to idle. The PCM saves the signal data after these conditions are met.