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Severity: Moderate Code type: Manufacturer-specific (Jeep)

Jeep P1391 Code: Intermittent Loss of CMP or CKP Signal

The PCM keeps briefly losing the cam or crank position signal, which can make your Jeep stumble or stall.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
Intermittent Loss of CMP or CKP Signal
System
Ignition & Misfire · Powertrain
Safe to drive?
With caution
Typical repair cost
$40–$550
DIY difficulty
Moderate

The PCM needs the crankshaft position (CKP) sensor for engine speed and piston position and the camshaft position (CMP) sensor to know which cylinder is on its compression stroke. P1391 is a Chrysler code used on Jeeps such as the Wrangler TJ, Cherokee XJ, Grand Cherokee ZJ/WJ and early Liberty: a failure counter goes up every time one of the two signals drops out while the other is still present, and the code sets when that counter reaches its limit on two consecutive trips. Unlike a hard sensor failure, the engine usually keeps running, but each dropout cuts spark and fuel for a moment. That's why owners describe bucking, a stumble over bumps or a stall that restarts fine a minute later.

Can I keep driving? You can usually drive it, but a stall in traffic or on a highway ramp is a real possibility, and the engine may not restart right away once the sensor is hot. Get it diagnosed soon and avoid long trips until then.

How to read P1391

How to read P1391P11233914
  1. P Powertrain
  2. 1 Manufacturer-specific
  3. 3 Ignition / misfire
  4. 91 Specific fault

Symptoms

  • Check engine light on
  • Momentary stumble, buck or cut-out, often over bumps or on turns
  • Engine stalls when hot and restarts after cooling a few minutes
  • Tachometer needle drops briefly while driving
  • Hard or long cranking
  • Misfire codes stored alongside

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Crankshaft position sensor failing when hot, or damaged wiring near the exhaust or bellhousing 34%
  2. Camshaft position sensor or distributor pickup failing (4.0 distributor-type sync sensor) 25%
  3. Low-quality aftermarket replacement sensor or wrong sensor spacing 14%
  4. Loose, corroded or oil-soaked sensor connector 12%
  5. Worn distributor/sync drive or oil pump drive gear allowing cam signal jitter 8%
  6. Weak ground, charging problem or PCM input fault 7%

Diagnosis step by step

Diagnostic flow: P13911Record freeze frame and relatedcodes2Inspect the crank sensor harness3Check the cam sensor and its drive4Wiggle-test while monitoring5Check the sensor supply and ground6Replace with quality parts and clear

  1. Record freeze frame and related codes

    Note engine temperature and rpm when P1391 set. A code that sets only when the engine is hot points to a heat-sensitive sensor; P0340 or P0335 stored alongside tells you which signal went missing entirely.

    Tools: OBD-II scanner

  2. Inspect the crank sensor harness

    On the 4.0 inline-six the crank sensor bolts to the top of the transmission bellhousing on the driver side, and its lead runs close to the exhaust. Look for melted insulation, a cable rubbing on the bellhousing and an oily or loose connector.

    Tools: Flashlight

  3. Check the cam sensor and its drive

    On the 4.0 the cam signal comes from a sensor in the distributor or sync drive on the passenger side. Remove the cap or sensor and check for oil inside, a cracked housing and side-to-side play in the shaft; excessive play makes the signal jumpy.

    Tools: Screwdriver, Flashlight

  4. Wiggle-test while monitoring

    With the engine idling, watch rpm and the sync status on a scan tool while you flex each sensor's harness and tap the sensor lightly. A dropout, stumble or loss of sync while you move a particular section finds the fault.

    Tools: OBD-II scanner

  5. Check the sensor supply and ground

    Both sensors are Hall-effect types that need a regulated supply (usually around 5 or 8 volts depending on the PCM) and a clean ground. Back-probe the connectors with the key on; a supply that dips or a ground with more than a few tenths of a volt drop causes intermittent signals.

    Tools: Multimeter

  6. Replace with quality parts and clear

    If a sensor is at fault, use an OE or high-quality replacement, and if the new sensor comes with a paper spacer or gap instructions, follow them exactly. Clear the code and drive a full heat cycle; P1391 needs two trips to set again, so drive a few days before calling it fixed.

    Tools: Torque wrench, OBD-II scanner

Where the parts are

Ignition: coils and spark plugs per cylinder – P13911234567
Ignition: coils and spark plugs per cylinder
  1. 1Ignition coil
  2. 2Spark plug
  3. 3Fuel injector
  4. 4Camshaft sensor
  5. 5Crankshaft sensor
  6. 6Engine computer (ECM)
  7. 7Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Repair sensor connector or heat-damaged wiring Yes $40–$220
Replace crankshaft position sensor Yes $90–$280
Replace camshaft position sensor or distributor pickup Yes $80–$260
Replace worn distributor or sync drive assembly No $200–$550

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Jeep

On Wrangler TJ and Cherokee XJ with the 4.0, P1391 with hot stalling is a well-known pattern, and many owners report the code returning after installing cheap aftermarket crank or cam sensors; quality sensors are worth the extra cost here.

Emissions / smog test impact

Will fail

P1391 lights the check engine light, so the vehicle fails an OBD emissions test in states that test it until the fault is fixed and the code has been cleared with monitors complete.

Frequently asked questions

Is it the cam sensor or the crank sensor?

The code doesn't say which. Freeze frame data, a companion code like P0340 or P0335 and a wiggle test usually narrow it down. On the 4.0, the crank sensor is the more common culprit when stalling happens hot.

Why does my Jeep stall hot and restart after it cools?

A failing Hall-effect sensor often loses its signal when it heats up and recovers as it cools. That hot-stall, cold-restart pattern is classic for a dying crank sensor.

I replaced both sensors and still have P1391. Now what?

Check the wiring and connectors again, especially the crank sensor lead near the exhaust, and question the quality of the new parts. On high-mileage engines, play in the distributor or sync drive can also make the cam signal erratic.

Can I clear the code and keep driving?

You can, but it will come back because the counter keeps running. The real risk is an unexpected stall, so don't ignore it for long.

Sources