P1398
Misfire Adaptive Numerator at Limit
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The P1398 code on Chrysler, Dodge, and Jeep vehicles means the PCM's misfire adaptive numerator has reached its learning limit. To understand this code, you need to know that the PCM uses the crankshaft position sensor to detect misfires by measuring tiny variations in crankshaft speed. Before it can detect misfires, the PCM must first 'learn' the normal mechanical variations in the crankshaft's toothed reluctor ring — no two engines are machined exactly alike. P1398 sets when these variations are too large for the PCM to compensate for.
This code often sets under specific driving conditions: when the engine coolant is below 75°F, after 50 seconds of runtime, with A/C off, at speeds above 36 mph. The PCM monitors the CKP signal during these conditions and sets the code if any target window varies more than 2.86% from the reference window. The practical effect is that the PCM may not be able to reliably detect misfires, and the engine may actually be misfiring without setting the typical P0300-series misfire codes.
Start diagnosis by checking the CKP sensor installation — make sure it's properly mounted with the correct air gap. Check battery terminals and charging system voltage, as a momentary voltage drop to the PCM can corrupt the learning process. Clear the code and perform a CKP sensor relearn procedure (many Chrysler vehicles require a specific drive cycle for this). If the code returns, a compression test can help determine if there's an underlying mechanical issue causing excessive crankshaft speed variation.
Severity
Symptoms
- •Rough engine idle or occasional misfires
- •Check engine light illuminated
- •Slightly reduced fuel economy
- •Engine may feel less smooth than normal at steady speeds
- •Possible slight vibration at idle
Likely Causes
The PCM learns the CKP sensor's target window variations to detect misfires. If one or more target windows vary more than 2.86% from the reference, the PCM cannot reliably calibrate its misfire detection algorithm.
An incorrect air gap between the CKP sensor and the reluctor ring, or a sensor that's loosening from its mounting, produces inconsistent signal patterns that exceed the adaptive learning limits.
Variations in cylinder compression from worn rings, leaking valves, or head gasket issues create crankshaft speed fluctuations that the PCM interprets as target window variations beyond its learning capacity.
A temporary drop in supply voltage to the PCM — caused by a weak battery, corroded terminal, or high electrical load — can corrupt the CKP learning process and push the adaptive numerator to its limit.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace and properly install the crankshaft position sensor with correct air gap
- Clean or replace battery terminals and check charging system voltage
- Perform a compression test to rule out base engine issues
- Clear the code and perform the CKP relearn procedure