P1391 Jeep Code Meaning, Symptoms, and Fix

P1391 is one of those fault codes you wouldn’t want to have. While its symptoms are only a few, its implications are pretty severe when left unaddressed, and potential causes are almost limitless. Furthermore, its triggers are highly dependent on application – something that this guide will help shed light on.

Trouble code P1391 usually pertains to an intermittent CMP/CKP signal, low input voltage to the glow plug or its circuit, or problems with a car’s wheel speed/G-sensor. Its symptoms include an illuminated CEL, erratic engine performance, no-starts, and white exhaust fumes.

This article won’t claim to offer cut-and-dry solutions for nipping the code in the bud. After all, P1391 is a blanket fault code and turns on and off like a light switch. But if you’re searching for a more extensive take on the issue, its symptoms, and causes, then stick around and keep on reading.

What the P1391 Code Means

P1391 is an error code predominantly referring to an issue with either your Camshaft Position (CMP) Sensor, Crankshaft Position (CKP) Sensor, or both. The drawback of other car makes and models is with the G-Sensor – “a vertical low acceleration sensor with gravity (G) detection.” There are peculiarities in how this fault code is defined, depending on the type and manufacturer of your vehicle.

Like in most automobiles, the Powertrain Control Module (PCM) monitors these sensors. A disagreement between the CMP sensor position and the expected CMP sensor level will trigger code P1391. But in some cases, the code could be set off by a jeopardized PCM or problems with wiring and connectors.

As already established, this fault code may be worded differently depending on the car manufacturer. However, it isn’t just the wording. Implicated components also vary depending on vehicle make. Below are examples of how the error code is defined for some of the biggest names in the market:

Vehicle	Error Code Reading
P1391 Dodge	CMP or CKP Signal Intermittent Condition
P1391 Ford	Glow Plug Circuit Low Input (Bank 1)
P1391 Honda	Wheel Speed Sensor 1 – G-Sensor Circuit Performance
P1391 Jeep	CMP or CKP Signal Intermittent Condition

Low Input Voltage

For automakers like Ford, Lincoln, Mazda, and Nissan, P1391 triggers when the PCM detects that the glow plug control circuit has a low input voltage. The “(Bank 1)” portion of the description refers to which bank of cylinders the control circuit is located. Since the definition of the error is quite self-explanatory, it’s safe to say the glow plugs themselves are rarely to blame for setting off the fault. Nonetheless, glow plug failure shouldn’t entirely be overlooked when determining the problem source.

G-Sensor Performance Problem

P1391 codes scanned from Acura, GMC, and Suzuki models (among others) pertain to a suspected defect with the wheel speed/G-sensor. However, it would be unwise to assume having a faulty sensor at the first display of this code since the latter is activated by unusual spikes in vertical acceleration – an outcome naturally caused by potholes, misfiring, or transmission line deviations.

That said, driving on difficult road conditions is almost guaranteed to set code P1391 off. Plus, the G-sensor shares the same PCM voltage reference and ground terminals with the A/C Pressure Sensor doesn’t make isolating its root cause any easier.

How Glow Plugs Work

Functionally, glow plugs are resistors whose heat is used for air-fuel mixture in diesel engines and helps efficient combustion. They mostly remain heated for a set period after start-up, tremendously helping during very cold or freezing weather. Hence, a diesel-fed engine is very unlikely to start if it has one or more failing glow plugs or when there’s something wrong with the glow plug control circuit.

These plugs are powered by a low voltage circuit energizing a heavy-duty electromechanical relay. The current fed to them only happens for a specific amount of time, and a timer cuts power to the relay after the OEM-set time has elapsed.

In normal circumstances, the engine would fire right up after this process. But if it doesn’t, the glow plug control circuit can be activated again, and heating of the glow plugs can continue.

The PCM monitors input voltage sent to the glow plug relay before and after, when the system is operable. When input voltage falls below spec, the PCM will set off the P1391 code alongside a warning light.

It is important to understand that older versions of glow plugs utilize 12 VDC current and heat up much slower than more recent ones. And new ones, while heating up faster, are controlled by dedicated control modules that employ pulse width modulation and supply 6 VDC current to the glow plugs to heat them individually.

Knowing this distinction and what system your car has will help in how you’d troubleshoot your glow plug, circuitry, and control module – should any of these components give you problems.

Camshaft and Crankshaft Sensors

Both CMP and CKP (or camshaft and crankshaft position) sensors serve as an informant and messenger to the PCM/ECM. They give your car’s ‘brain’ crucial information on camshaft speeds and the like that helps the latter determine the proper timing of ignition and fuel injection that the mill requires.

The inability of these sensors to send accurate data (or any information at all) to the PCM would result in a system malfunction. Of course, these sensors fail over time or due to normal wear and tear. But untoward accidents or a manufacturing error can also be causes.

DTC P1391 Symptoms

Your vehicle may or may not trigger the P1391 code depending on the situation. A DRB-III scan tool like a LAUNCH X431 PRO3S+HDIII Diesel/Gasoline Bidirectional Diagnostic Scan Tool (view on Amazon) would be helpful. However, it would be better if you’re aware of signs that tell you of an impending glow plug or sensor issue beforehand. That said, let me loop you in on a few of those symptoms:

1. Check Engine Light or CEL is on

Like most other DTCs or error codes, a faulty CMP/CKP sensor triggers the Check Engine Light. And just like the others, it isn’t automatically the root cause of an illuminated CEL. You’ll need to determine what vehicular system or function has led to the sensor becoming defective and go from there.

Using a high-spec OBD-II scan tool should bring up several fault codes if there are multiple reasons behind this indicator. Relevant errors like P0340, P0344, P1120 to P1698 are likely to appear. However, it’s also possible that the CMP/CKP sensor is either gummed up or compromised.

2. No-Starts and Power Loss

Although very common, starting difficulties are actually an aftermath of ignoring earlier signs of the P1391 code. Specifically, vehicle owners have observed that no-start issues occur when the engine is cold or not yet fully primed. This makes perfect sense, as a weakened camshaft wouldn’t be able to transmit sufficient signal to the vehicle’s ‘brain.’ It also strongly suggests a potential problem with your crankshaft sensor when the no-start issue is characterized by extended cranking times.

At first, it may only be the crankshaft or camshaft at fault. But suppose the problem is left unattended too long. In that case, its weakened state eventually translates into the quality of the signal received by the PCM – even failing to reach the latter in worst-case scenarios and resulting in complete, abrupt power loss.

You would want to avoid this costly (not to mention dangerous) situation at all costs. A keen eye for discrepancies in your vehicle’s ‘body language’ and promptness in replacing defective sensors with OEM variants are proven to mitigate this issue.

3. Frequent Stalling or Rough Idling

Spotty or unpredictable engine performance such as rough idling or slow acceleration is a telltale sign of a failing CMP sensor. While these symptoms escalate gradually, it would be much better to address them early on and have the CMP sensor checked by a professional mechanic and replaced at once (when needed).

Tarrying on these can lead to engine cutouts or, worse, your vehicle abruptly dying in the middle of nowhere. But be warned, these symptoms also point to other car issues, such as fuel starvation and the like – hence, careful diagnosis is advised.

4. White Exhaust Fumes

Primarily associated with a coolant leak, this symptom also happens when you start your car on cooler days. It does go away immediately as soon as your engine warms up. But if it persists (and does so repeatedly), you may be looking at a P1391 code you just haven’t scanned yet.

If a glow plug relay is compromised, it wouldn’t be able to produce the needed heat for your vehicle’s air-fuel mixture. As a result, condensation occurs, which causes steam or white smoke to come out of your tailpipe. Just make sure there isn’t any accompanying sweet smell, as that would mean a coolant leak instead.

What Causes the P1391 Jeep Code?

Unlike its number of symptoms, many factors can cause your vehicle’s CKP/CMP sensors to go bad, and it’s not always the fault of the glow plugs or the PCM. In some cases, the sensors are just severely worn. At other times, the culprit is your car’s electrical.

Having an immediate look at these two is almost an expectation. But if you think it’s something else, consider the below when narrowing the root cause of a P1391 code:

P1391 Jeep Code Causes

• Faulty Crankshaft Position (CKP), Camshaft Position (CMP), or G-Sensor
• Poor electrical connection of the CKP/CMP circuitry; open/shorted CKP/CMP sensor harness
• (Tip: The terminals in the connector itself tend to spread – meaning that if the wiring harness female terminal gets too loose, it wouldn’t be able to reliably connect with the sensor male terminals. To inspect, hack out one of the male electrical terminals from your old sensor, hold that terminal with your fingertips, and test physical resistance and friction by carefully inserting each into the harness side connectors cavities. Otherwise, splice a new connector.)
• Burnt/corroded/damaged/disconnected wiring and/or connectors (including PCM)
• Wiring wiggling leading to intermittent CKP/CMP signal loss
• Carbon-filled IAC (Idle Air Control) valve – common for Dakota Dodge
• Dirty reluctor pickup for the CMP sensor
• Metal debris or other obstruction stuck in the CKP sensor
• Stretched/loose timing belt or chain
• Out-of-sync CMP sensor
• Damaged crankshaft tone wheel, flex plate, or pulse ring
• CKP sensor physically in contact with the flywheel
• Defective glow plug relay/timer (common with Ford, Lincoln, Mazda, Mercury, and Nissan models)
• Damaged AC Pressure Sensor for Honda, Buick, Chevrolet, and Cadillac (to name a few)
• Irregular CMP signal lab scope pattern
• Out-of-spec sensor or sensor voltage
• Blown fuses
• Low battery voltage
• Other faulty sensors (usually ECT or engine coolant, oil/air intake temperature sensors)
• Defective oxygen sensors; physical damage to components (for example, catalytic converter) within its surrounding area
• High PSP input to the PCM
• Dismissing early indicators of defective sensors or low coolant levels
• Intermittent firing of coils and fuel injectors (resulting from the PCM getting a weak signal)
• Poor vehicle upkeep or a disregard for regular inspection and periodic change of fuses, spark plugs, wiring, connectors, etc.
• Failed Powertrain Control Module (PCM) or glow plug control module (a rare occurrence)
• Outdated PCM software

Distributor Problems

Based on forums, the P1391 Code on Jeeps is usually triggered when the distributor/cam sensor assembly is phased incorrectly. A Jeep owner may have their engine upgraded to a long block but still paired their old distributor with it – hence, an off timing. Or there may be excessive wear on the teeth of the distributor (a typical case with high-mileage Jeeps).

Luckily, the first scenario is an orientation problem and can be resolved by putting in an alignment pin while ensuring the distributor is put on the proper TDC. The second one, on the other hand, warrants a replacement. Also, consider that since the PCM controls all timing, a distributor in the right window but with a flawed PCM would yield the same code.

Make sure that you opt for genuine OEM Jeep parts whenever possible. Not to undermine aftermarket replacements, but some of them are either poorly manufactured, made of inferior materials, or failed/out-of-spec right out of the box. Additionally, use a DRB-III scan tool or an oscilloscope (view on Amazon) when checking for synch. Never jump into ordering ‘suspected’ defective sensors without doing this first.

P1391: Diagnosis and Fix

This section only covers diagnosis for suspected glow plugs, sensors, and low input voltage errors. If you’re dealing with any of the other potential causes listed above, refer to your service manual for guidance on correct procedures:

1. Check Related TSBs

Check related TSBs (Technical Service Bulletins) for existing issues specific to your vehicle’s glow plug circuitry or CMP/CKP/G-sensor.

2. Record Fault Codes

Using an OBD-II scanner, record all fault codes that occur alongside the P1319 code, as well as freeze-frame data. If there are other error codes, make sure to resolve them first. Clear the other codes in the order they were stored before diagnosing P1391. A higher-spec OBD-II or DRB-III tool is much preferred in completing this step.

3. Examine Parts

Examine the wiring and connectors for burnt marks, arcing, rusting, damage, fraying/saturation, or disconnection – lest you’ll need to deal with out-of-spec resistance/ground readings later on. Refer to your owner’s manual for color-coding and routing specifications to make locating relays, fuses, or other potentially implicated components easier.

Also, note that spotting any of these flaws requires immediate part replacement. Once you’ve ascertained that your car’s charging/electrical system is fully functional, proceed with looking into the suspected faulty sensor or glow plug circuit.

4. Perform Checks

Perform resistance and continuity checks on all fuses/fusible links. Additionally, check for ground and reference voltage as needed. Check actual readings against OEM specs in the manual, and replace any relay found defective.

To ensure no damage, disconnect relevant sensors from the PCM before conducting these checks. After relay testing or replacement, re-scan the system and see whether the P1391 code clears or persists.

5. Test Sensors

In cases where the suspected glow plug relay checks out, but P1391 doesn’t clear, you’ll need to determine what controls the glow plugs. Ensure it is working as intended, whether it’s the oil temperature or engine coolant temperature sensor.

If not, consult your owner’s manual for correct steps on testing the malfunctioning/suspected sensor. OEMs provide a temperature-to-resistance chart that should help you achieve the most reliable results.

6. Test Wiring

Test the wiring leading to the relay if the code is persistent. Be surgical when you do this step, strictly adhering to instructions in the manual as you go along. Note that values for currents and resistances entering a glow plug relay vary immensely and can be easily misdiagnosed.

To avoid the latter, make sure to use a high-spec digital multimeter (view on Amazon) while ensuring the current is at the correct voltage at the time.

7. Repeat Tests

At this point, you should already have determined a part needing repair or fully resolved the issue. But if the P1391 code still shows up despite no visible damage and the above steps all checking out, perform the same tests (ground, resistance, continuity, etc.) but on all associated wiring.

Cars with dedicated control modules for glow plugs need to have their controllers disconnected first before carrying on with tests. Results from these tests will tell you if further tests or repairs are required.

8. Check Voltage

Locating the glow plug control circuit with low input voltage can be especially difficult for recent-year vehicle models. However, for cars with older systems, the wiring/continuous conductor found between the simultaneously-heated glow plugs and the relay is the likely spot.

Check for voltage while energizing the glow plug relay. Readings should match OEM values. If they don’t, you may have a poor connection where the relay cable attaches to the conductor or an intermittent system fault.

9. Test Glow Plug Leads

Systems with dedicated control modules are self-diagnosing by design. Unfortunately, generic code readers often cannot access information saved in these controllers. That said, testing the glow plug leads on these systems for resistance or voltage is necessary but warrants strict adherence to instructions in the owner’s manual. If done haphazardly, the procedure can damage the glow plug control module.

(Tip: After identifying a defective glow plug lead, it’s best to replace the leads as a complete set)

10. Test Glow Plug Resistance

If all other suspected components check out, test the resistance of the glow plugs. Although this is often a last resort to determining the problem source of code P1391, it isn’t one to be taken off the list. Since glow plugs are self-limiting, expect a sharp drop in resistance during testing.

However, it’s not only these so-called drops but also the elapsed time before they happen that you need to pay mind to. These timing discrepancies are a sure-fire indication of a fault in the control system. In like manner, range-exceeding variances in resistance and amperage draw mean a defective/malfunctioning glow plug.

Repair Costs for Code P1391

Component	Estimated Cost
Crankshaft Position (CKP) Sensor	$120 – $300
IAC (Idle Air Control) Valve	$120 – $500
Camshaft Position (CMP) Sensor	$130 – $325
Glow Plug & Related Components	$150 – $650
AC Pressure Sensor	$160 – $175
Catalytic Converter	$200 – $3,750
Wheel Speed/G-Sensor	$300 – $700
Timing Chain	$400 – $1,800
PCM or ECM	$800 – $2,000
Other Faulty Sensors	$100 – $300

A Few Notes

• Vehicles from California or any other ‘Green States’ require glow plug control systems to have self-test diagnostic function not present in other automobiles. Therefore, it is a must to refer to your service manual for correct procedures to perform tests on these additional components.
• Ensure that the engine is cold before starting a diagnostic procedure on the P1391 or any other glow plug-related error code – since the control system deactivates when engine temperature exceeds a specified value. Never test the control system when the engine is hot.
• Always refer to relevant technical information for your car and never assume probable causes of low input voltage. To side with caution, vehicle owners who aren’t professional mechanics or aren’t mechanically savvy should never attempt to work on a P1391 or glow plug-related code without a handy and comprehensive service manual.
• Volkswagen applications with TDI (Turbocharged Direct Injection) engines may flash a glow plug warning light. However, this doesn’t necessarily indicate a problem in the control circuit. VW engineers designed the former to double as a blown bulb indicator. That said, it is advisable to check if there are any blown bulbs before looking into a glow plug-related fault.

Conclusion – P1391 Symptoms & Fix

Whether the culprit is your CMP/CKP sensors, glow plug circuit, speed sensors, or the PCM, it pays to have a DRB-III scan tool, multimeter, and your service manual close by. Anyone who spins a wrench will advise you never to take on diagnostics if you’re inexperienced. And if you’re up to the task skill-wise, they’d probably sound like broken records telling you the importance of opting for OEM sensors and replacement parts.

If all else fails, seek the assistance of a professional mechanic in getting to the bottom of this fault. Hopefully, this article gets to answer your questions about the P1391 code and helps you resolve it if you’re in the middle of fixing one!