P1391 – Glow Plug Circuit Low Input Bank 1 (Ford, Lincoln, Mercury, Mazda, Nissan)


By Reinier (Contact Me)
Last Updated 2016-11-14
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P1391 P1391 – Glow Plug Circuit Low Input Bank 1 (Ford, Lincoln, Mercury, Mazda, Nissan)
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Manufacturer Specific Definitions for P1391

MakeFault Location
AcuraG-sensor - performance problem
AudiCamshaft position (CMP) sensor 2/ Bank 2 - short to ground
BuickWheel Speed Sensor 1 G-Sensor Circuit Performance
CadillacWheel Speed Sensor 1 G-Sensor Circuit Performance
ChevroletRough road sensor -signal malfunction
ChryslerCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
CitroenGlow Plug Circuit Low Input (Bank #1)
DaewooRough road G-force sensor – implausible signal
DodgeCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
EagleCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
FordGlow Plug Circuit Low Input Bank 1
GmWheel Speed Sensor 1 G-Sensor Circuit Performance
HondaG-sensor - performance
IsuzuG-force sensor – performance
JeepCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
MitsubishiIntermittent Loose Of Camshaft Position Or Crankshaft Position Sensor
PeugeotGlow Plug Circuit Low Input (Bank #1)
PlymouthCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
RamCrankshaft position (CKP) sensor/camshaft position (CMP) sensor -intermittent signal
SaabCylinder 1, misfire detected – fuel level low
SuzukiG Sensor Rough Road Rationality
VolkswagenCamshaft position (CMP) short to ground

What Does Code P1391 Mean?

OBD II fault code P1391 is a manufacturer specific code that is defined by car makers Ford, Lincoln, Mercury, Mazda, and Nissan as “Glow Plug Circuit Low Input (Bank #1)”. On these applications, code P1391 is set when the PCM (Powertrain Control Module) detects a low input voltage in the glow plug control circuit. “Bank 1” refers to the bank of cylinders that contains cylinder #1 on diesel engines with two cylinder heads. Note that this code refers specifically to a low input voltage in the glow plug control circuit, and as such, the glow plugs themselves are rarely implicated in this code, although in some cases, glow plug failure can contribute to code P1391 being set.

Glow plugs are essentially resistors that heat up when a current is applied to them, with the resulting heat being used to heat the air/fuel mixture in diesel engines to aid in the combustion of the air/fuel mixture when the engine is cold. On most applications, the glow plugs will remain heated after start-up for a set period to aid combustion, especially during very low ambient temperature conditions. Note therefore that with very few exceptions, a diesel engine can generally not be started in low temperatures when the glow plug control circuit is inoperative, or if one or more glow plugs have failed.

In terms of general operating principles, when the ignition of a diesel-powered vehicle is turned on, a low voltage circuit energizes a heavy-duty electro-mechanical relay that then feeds power to the glow plugs simultaneously with current drawn directly from the battery. However, the amount of time that current is fed to the glow plugs is controlled by a timer that cuts power to the relay after the time set by the manufacturer has elapsed. If the engine does not start after this time, the glow plug control circuit can be activated again to continue heating of the glow plugs.

While the above describes the working principles of older glow plug control systems, new developments in the field have produced glow plugs that heat up faster than older versions, while consuming less current. On these systems, the glow plugs are controlled by dedicated control modules that employ pulse width modulation to heat up each glow plug individually. This type of system typically supplies 6 VDC current to the glow plugs, in contrast to the 12 VDC current used by older systems.

Regardless of the design of the control circuit though, the PCM monitors the input voltage both before and after the glow plug relay when the system is in operation, and when it detects an input voltage that is lower than expected, it will set a trouble code and illuminate a warning light.

The image below shows the basic construction of a typical glow plug.


What are the common causes of code P1391?

Some common causes of code P1391 could include the following-

  • Defective glow plug relay/timer
  • Defective engine coolant /oil temperature/intake air temperature sensor, depending on the application
  • Low battery voltage
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • Open circuits
  • Failed PCM or glow plug control module. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced

What are the symptoms of code P1391?

Common symptoms of code P1391 could include the following, but note that the severity of one or more symptoms could vary between applications-

  • No start condition when the engine is cold
  • Engine may not idle
  • Idle may be rough or erratic
  • Extended cranking times when the engine is cold, or has partially cooled down
  • White smoke from the tail pipe may be present upon start-up and during the warm-up cycle
  • Loss of power may be present following a cold start and during the warm-up period. Note that in some cases the loss of power may be slight, while in others, it may be severe enough to render the vehicle undriveable

How do you troubleshoot code P1391?

NOTE #1: Exhaust emission control regulations in some states, California in particular, require that glow plug control systems incorporate a self-test diagnostic function to identify failed, or failing glow plugs, and that the diagnostic function must illuminate a warning light if such failed or failing glow plugs are likely to increase, or affect exhaust emissions negatively. Therefore, vehicles from California, or from the so-called Green States, will have components in their glow plug control systems that are not present on vehicles that do not have to comply with California, or “Green State” regulations. Consult the manual for these applications for information on testing procedures for the additional components.

NOTE #2: Always make sure that the engine is cold before starting a diagnostic procedure for this, or any other glow plug related code since the control system is deactivated if the engine temperature exceeds a specified value. In practice, this means that the control system cannot be properly tested when the engine is hot.

WARNING: Since the power requirements of glow plugs vary greatly between applications, no assumptions about the probable cause of a low input voltage should be made without reference to the relevant technical information for the application being worked on. Non-professional mechanics in particular are strongly urged NOT to attempt a diagnosis/repair of P1391 (or any other glow plug related code) if the relevant/required repair manual or technical information is not available. Note that any diagnostic procedure(s) for this code that is based on guesswork and false assumptions has the potential to cause extensive damage to the vehicle’s electrical system.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on. If any other codes are present along with P1391, resolve these codes in the order in which they were stored before proceeding with the diagnostic procedure for P1391, and especially if the other codes indicate low (or abnormal system voltages) or if additional codes indicate problems with the alternator, battery, or charging system in general.

NOTE: Be aware that a flashing glow plug warning light on VW applications with TDI engines does not necessarily indicate a problem in the glow plug control circuit. For reasons that are not entirely clear, VW engineers have decided that the glow plug warning light should do double duty as a “blown bulb” indicator, so always make sure that a glow plug related fault actually exists when the warning light starts flashing.

Step 2

Once all other codes had been resolved and it is certain that the3 applications’ charging/electrical system is fully functional, consult the manual to locate the glow plug relay, fuses, and other components that are likely to be implicated.

Check all fuses and/or fusible links for resistance and continuity, and replace all that are suspect, or proven to be defective. Clear all codes, and retest the system to see if the code returns.

Step 3

If the code persists, locate the glow plug relay, and consult the manual on the correct procedure to test its resistance. Compare the obtained reading with the manufacturer’s specifications, and replace the relay if it does not comply with specified values. Clear all codes after testing or replacing the relay, and rescan the system to see if the code returns.

WARNING: Some relays require that wires and cables be removed either to test, or to replace the relay. Be aware that in most cases, some of the cables will be live, so exercise extreme caution when removing cables to prevent short circuits that could cause fires and/or damage the vehicles’ electrical system.

Step 4

If the code does return but the relay checks out OK, consult the manual to determine if the glow plugs are controlled by the engine coolant temperature, or by the oil temperature. In both cases, it is necessary to check if the relevant sensor is working as intended, since a malfunctioning sensor can cause or contribute to setting code P1391. Consult the manual on the correct procedure to test the implicated sensor, but be aware that to get the most reliable result, you need to have access to the manufacturers’ temperature-to-resistance chart for that sensor. Follow the testing procedure in the manual exactly, and replace the sensor if it does not comply with specified values.

Clear all codes after the sensor replacement, and rescan the system to see if the code returns.

Step 5

If the code still persists, prepare to test the wiring leading to the relay, but be aware that to prevent accidental short circuits during this step, it is important to follow the instructions in the manual EXACTLY.

The values for the currents (and their resistances) that enter the glow plug relay vary greatly between applications, which means that to avoid misdiagnoses, it is necessary to use a top quality digital multimeter for this step. The manual will describe the testing procedure in some detail, but this step generally involves testing the current that enters the “low” side of the relay. In addition, the current needs to be at the correct voltage for the prescribed time, although timer failure is usually indicated by a code other than P1391.

Step 6

If the input current does not match the specified value, the most likely cause is either a high resistance, or a poor ground connection.

If the required input voltage is not present, prepare to perform a thorough visual inspection of all associated wiring, but be aware that it may be necessary to remove parts of the dashboard, and/or trim panels to gain access to all relevant wiring.

Nonetheless, look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors, and make repairs as required. Clear all codes after repairs are complete, and rescan the system to see if the code returns.

Step 7

If no visible damage to the wiring is found but the code persists, prepare to perform ground, resistance, and continuity tests on all associated wiring. Consult the manual to determine the location, routing, color-coding, and function of each wire, but take great care not to insert a test probe into any terminal, or allow it to touch any pin other than the terminal or pin indicated in the manual- allowing this to happen can cause serious, if not always fatal damage to the electrical system.

If the application is fitted with a dedicated control module for the glow plugs, be sure to disconnect the module from the system to prevent damage to the module during this step. Compare all obtained readings to the values stated in the manual, and if discrepancies are found, make repairs as required, or replace wiring as required to ensure that all electrical values fall within the manufacturer’s specifications.

Step 8

Assuming that all components are in working order and that all repairs had been performed to industry standards it is unlikely that the code will return, although it is not altogether impossible, since the definition of code P1391 does not indicate exactly where in the control circuit the “low input” might be.

On older systems where all the glow plugs are heated simultaneously, one likely spot is the wiring between the relay and the glow plugs, rather than in the glow plugs themselves. On these systems, the glow plugs on a bank of cylinders are all connected to each other with a continuous conductor, with the connector being connected to the relay with a single wire or cable. Therefore, if the glow plug relay is energized, the entire conductor should carry the specified glow plug input voltage for that application.

So, apply power to the glow plug relay, and check the voltage in the conductor by placing the red probe of the multimeter on the conductor and the black probe on a suitable ground, such as the engine block. If the displayed value deviates from the specified value, check the connection where the cable from the relay attaches to the conductor. Look for signs of arcing, burning, or discoloration that would indicate a poor connection, and hence a high resistance. Make repairs as required, and recheck the voltage in the conductor after repairs are complete to verify that the repair was successful.

Clear all codes, and rescan the system to see if the code returns. If the code does return or persists, it is likely that there is an intermittent fault present in the system. Note though that intermittent faults can be extremely challenging to find and repair and in some cases, it may be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Step 9

On systems that use dedicated control modules, each glow plug is provided with its own lead, much like a plug lead on a gasoline engine. However, testing these leads for high resistance, and hence low voltage, needs to be performed in strict accordance with the instructions in the manual to prevent damage to the controller on the one hand, and to obtain reliable and accurate results on the other.

Note also that even though glow plug control modules perform self-test diagnostic functions, generic code readers can often not access this information. In practice, this means that if this diagnostic information is not available, you need to follow the instructions on testing the glow plug leads exactly to avoid damaging anything.

Follow the instructions provided carefully, but make notes of the resistance of each lead; all leads should have the same or very close to the same resistance. Compare the resistance values to those stated in the manual, and replace all leads that do not conform to specifications. Clear all codes after replacing leads, and rescan the system to see if the code returns or persists.

If the code does return or persists, it is likely that there is an intermittent fault present in the system. Note though that intermittent faults can be extremely challenging to find and repair and in some cases, it may be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

NOTE: While it is sometimes possible to replace only the defective lead(s), the better option is to replace the leads as a complete set.

Step 10

As a final step, consult the manual on the correct procedure to test the resistance of the glow plugs themselves, since in some cases, differences in resistance and/or current draw can set trouble codes, or contribute toward setting code P1391.

Note that glow plugs are self-limiting in the sense that their internal resistance drops off sharply as they heat up. This is normal, so be prepared for the drop but take note of the time that elapses before it happens. Glow plug operation is set to a time period specified by the manufacturer, and if the resistance drops off too soon or too late, there is a fault in the control system.

Nonetheless, take careful note of each glow plug’s resistance and amperage draw, which should be the same for all glow plugs. The manufacturer may allow small differences, but any reading that exceeds the range specified in the manual is indicative of a defective glow plug. Replace any glow plugs that do not match the manufacturers’ specifications closely.

Clear all codes after repairs/replacements are complete, and rescan the system to see if the code returns. At this point, the code is unlikely to return, but if it does, suspect an intermittent fault to be the cause, rather than a failed or failing control module.

Codes Related to P1391

  • P1392 – Relates to “Glow Plug Circuit High Input (Bank #1)”

NOTE: Many manufacturers have assigned different definitions to code P1391. Below are some examples-

  • Acura – “G-Sensor Performance”
  • Chrysler/Dodge/Jeep/Mitsubishi – “Intermittent loss of CMP or CKP”
  • GM – “Wheel Speed Sensor 1 – G – Sensor Circuit Performance”
  • Volkswagen/Audi/Volvo – “Camshaft Pos. Sensor, (Bank2) Short to Ground”

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