P0380 – Glow plugs, circuit A -malfunction


By Reinier (Contact Me)
Last Updated 2018-02-07
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P0380 Glow plugs, circuit A -malfunction
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Wiring, glow plug relay, fuse, glow plugs, ECM

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What Does Code P0380 Mean?

OBD II fault code P0380 is a generic code that is defined as “Glow plugs, circuit A –malfunction”, and is set when the PCM (Powertrain Control Module) detects a general malfunction or failure in the glow plug control circuit on a diesel powered application. Note that while “Circuit A” refers to a particular part or section of the glow plug control circuit, car manufacturers do not always number circuits or parts of circuits in the same way. For this reason, it is important to refer to the manual for the affected application to determine which part of the glow plug control circuit is labelled “A” on that application. Failure to do this could result in a misdiagnosis, and the unnecessary replacement of parts and components.

The purpose of glow plugs on diesel engines is to provide a source of intense heat to assist in the combustion process of the air/fuel mixture when the engine is cold, or below a minimum threshold temperature. Typically, the glow plug control circuit consists of the-

  • battery
  • the ignition switch
  • one or more relays
  • one or more fuses and/or fusible links
  • a dedicated engine coolant sensor
  • wiring and connectors
  • warning and/or indicator lights to indicate the status of the glow plug control circuit
  • the glow plugs themselves

Depending on the application, the circuit could also include a timer, or on most modern applications, a dedicated, stand-alone glow plug control module.

During normal operation of a diesel engine, combustion of the air/fuel mixture is accomplished by heating the mixture adiabatically, i.e., by heating the mixture through compression when the pistons are moving upwards during their compression strokes, and provided that the engine and all the components of the fuel injection system are both in good mechanical condition, the air/fuel mixture will combust completely. Note though that the high speed at which the pistons move upward during the compression stroke greatly assists in heating the air/fuel mixture when the engine is running.

However, at low ambient temperatures, it can be very difficult to ignite a diesel fuel/air mixture at start up because not only is combustion less efficient at low temperatures, but also because the air fuel mixture may not be compressed sufficiently to initiate combustion at engine cranking speeds. Thus, to eliminate these problems, diesel engines use electrically heated glow plugs to provide a source of heat in the pre-combustion chamber of each cylinder to assist in igniting the air/fuel mixture at the compression peak.

In terms of operation, glow plug control starts when the ignition is turned on. At this point, the PCM receives input data on the engine coolant temperature (from a dedicated coolant temperature sensor), which on most applications, has to be below about 1040F (400C) for the glow plug circuit to be activated. If the PCM determines that the engine coolant is below a pre-defined minimum threshold, it will feed power to a timer on some older applications, which in turn, will feed battery power to the glow plugs via a dedicated relay.

NOTE: On some older, non-monorail injection applications that still use glow-plug timers, the glow plugs will be fed battery power only for a predetermined time, (or until the engine coolant reaches a predetermined minimum temperature), which usually sufficed to get a cold engine running smoothly. However, on monorail injection systems where the fuel injectors are controlled electronically, the PCM may adapt both the injection timing and the injection pulse width during the time the glow plug control circuit is in operation, which is usually until the engine coolant reaches a predetermined temperature before the glow plug control circuit is deactivated to remove power from the glow plugs.

It must be noted however, that regardless of the ambient temperature, the glow plug control circuit will not be activated if the engine coolant temperature exceeds a predefined minimum threshold. This is to both extend the life of the glow plugs, and to prevent the air/fuel mixture from detonating prematurely if the glow plugs are heated while the engine is hot.

From the above it should be obvious that the glow plugs and their control circuits/mechanisms are critically important parts/components of a diesel engine, because if the control circuit fails, there is a high likelihood that if the engine is cold, the engine cannot be started, or can only be started with difficulty. Therefore, if the glow plug control circuit fails, the PCM (or other control module) will set code P0380, and may also illuminate a dedicated warning light.

Where is the P0380 sensor located?

The image above shows a simplified schematic of a typical glow plug control circuit that does not incorporate a dedicated control module, but bear in mind that the actual location of the components shown here vary greatly between applications.

Also, note that the glow plugs themselves may or may not be easily accessible on all applications. While the glow plugs are generally screwed into the cylinder head much like spark plugs on gasoline engines, on some modern applications it may be necessary to remove, or partially disassemble unrelated engine components to gain access to the glow plugs. For this reason, it is important to refer to the manual for the affected application to locate and identify parts/components/wiring in the glow plug control circuit/system correctly to avoid a misdiagnosis.

What are the common causes of code P0380 ?

Some common causes of code P0380 could include the following-

  • One or more defective glow plugs, but note that it is highly unusual for all the glow plugs to fail at the same time
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Note that since glow plugs are connected in parallel on applications that do not use dedicated glow plug control modules, loss of ground or battery power can affect all the glow plugs on a cylinder bank.
  • Blown fuse(s)/fusible link(s)
  • Defective glow plug relay
  • Defective ignition switch
  • Defective engine coolant temperature sensor
  • Defective timer (where fitted)
  • Defective glow plug control module (if fitted). Note that this is fairly common on applications that use stand-alone glow plug control modules
  • Failed or failing PCM. Note that this is a rare event, and therefore the fault must be sought elsewhere before the PCM is condemned

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