Crankshaft Position System Variation Not Learned

What P0315 means

Every crankshaft and CKP sensor has tiny machining and mounting tolerances, so the gaps between the reluctor teeth are never perfectly even. The crankshaft position variation-learn feature measures those slight reference-period errors and stores a set of correction values in the engine control module (ECM). With those corrections in place, the ECM can tell a real combustion-event slowdown from normal tooth-spacing variation, which lets it detect misfire accurately across a much wider range of engine speed and load. Those variation-compensation values are only saved after the learn procedure has been run and the ignition has been switched OFF for at least about 30 seconds so the module can power down and commit them to memory. P0315 sets when the ECM finds no valid variation values stored, or when a proper power-down did not occur after the learn procedure so the values were never saved. It is a Type A DTC, meaning the check-engine light comes on the first time the fault is detected. Because the electrical circuits and the sensor itself can be perfectly healthy, this is a missing-calibration condition, not a broken component — the standard fix is to perform (and correctly save) the relearn, not to replace parts.

Symptoms

• Check-engine light on immediately after a PCM/ECM reflash, module replacement, CKP sensor replacement, or timing-related engine work
• P0315 stored on its own with no driveability complaint, since the engine still starts and runs normally
• A companion random-misfire code such as P0300 may set because the misfire monitor is running without valid variation corrections
• Occasional false or erratic misfire counts reported by a scan tool even when the engine feels smooth
• The MIL returns shortly after a scan-tool code clear if the relearn was never performed or never saved

Common causes

• The crankshaft position system variation learn was simply never performed after a repair that requires it (most common)
• The learn was started but not completed, or the ignition was not left OFF for at least ~30 seconds afterward, so the values never saved to ECM memory
• ECM/PCM reflash, reprogramming, or replacement that erased the previously stored variation values
• Crankshaft position sensor replacement, timing chain/sprocket service, or other work that changes the crank-to-sensor relationship and invalidates the old learn
• A weak or low battery, or a poor charging condition, that prevents the relearn from completing correctly (the procedure requires a fully charged, load-tested battery)
• A replaced or repaired ECM that cannot retain the learned values, requiring module replacement only after the relearn repeatedly fails

Severity & driving advice

Severity: Moderate — The engine runs normally, but until the relearn is done the misfire monitor is compromised and a genuine misfire could go undetected.

Can I drive? The vehicle drives normally with P0315, so short-term driving is generally fine. The catch is that misfire detection is degraded until the CKP variation relearn is completed, so schedule the relearn promptly rather than ignoring the light.

Diagnostic approach

1. Confirm the code and read the history — Read P0315 with freeze-frame and note any companion codes such as P0300 (random misfire). Ask what work was done just before the light appeared: a PCM reflash, ECM swap, CKP sensor replacement, or timing work all wipe or invalidate the stored variation values and are the usual trigger. Recognize this is a missing-calibration code, not a wiring fault, so no circuit testing is needed first.
2. Verify the battery and charging system — The variation-learn procedure requires a battery that has passed a load test and is fully charged, because the relearn will not complete reliably on a weak battery. Confirm state of charge and connections before attempting the learn, since a marginal battery is a common reason the procedure fails to save and P0315 keeps returning.
3. Bring the engine to operating temperature — Start the engine and let it reach normal coolant temperature. The ECM will only run and store the variation learn once the engine is fully warmed up and idle conditions are stable, so a cold engine will not complete it. Address any active engine faults first — the relearn should be run on an otherwise healthy engine.
4. Run the scan-tool Crankshaft Position System Variation Learn — With a capable scan tool, select the CKP System Variation Learn (crank relearn / CASE relearn) special function and follow its prompts. On GM 3.6L engines this typically commands a controlled wide-open-throttle acceleration followed by a closed-throttle deceleration so the ECM reads crank-speed variation during decel fuel cutoff (fuel injectors off while the engine coasts down from high rpm toward about 1000 rpm). The tool reports when the values are learned; hold the vehicle safely on a lift or a closed course during this step.
5. Power down so the values save, then verify — After the tool reports the learn is complete, turn the ignition OFF and wait at least about 30 seconds so the ECM powers down fully and commits the variation values to memory — skipping this leaves them unsaved and re-sets P0315. Restart, operate within the running conditions, and confirm P0315 does not return and no P0300 sets.
6. Replace the ECM only if the relearn will not save — If the battery is good, the engine is warm, and the learn has been performed correctly with a proper power-down but P0315 still sets, the ECM is not retaining the values. Per the factory verification path, replace the engine control module (with correct programming and setup) as the last step — only after the relearn itself has been ruled out.

FAQ