P0381 – Glow plug warning lamp -circuit malfunction

By Reinier (Contact Me)
Last Updated 2018-07-27
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0381 Mean?
2. Where is the P0381 sensor located?
3. What are the common causes of code P0381 ?
4. Get Help with P0381

What Does Code P0381 Mean?

OBD II fault code P0381 is a generic code that is defined as “Glow plug warning lamp – circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects an electrical failure, defect, or malfunction in the glow plug control circuit that prevents the glow plug warning lamp from illuminating. Note that this code only affects applications with diesel engines.

The function of glow plugs on diesel engines is to assist with the initial ignition of the diesel fuel /air mixture when the engine is cold, with “cold” referring to a condition where the engine coolant temperature is below a predefined limit, typically about 100⁰F.

In terms of operation, the glow plugs are fed with power from the battery when the ignition is turned to the “ON” position. On modern systems, the heating of the glow plugs is controlled by a dedicated glow plug control module that has the ability to sense the electrical resistance of each individual glow plug, and to modulate the power supply to each glow plug individually in order to heat all the glow plugs to the same temperature in the same amount of time.

In practice, all diesel vehicles have a dedicated warning light in the instrument cluster that illuminates when the glow plugs are being heated, which usually takes only about two, or sometimes three seconds. This is to alert the driver that the glow plug heating system is in operation, and in a fully functional system, the warning light is extinguished automatically either when the glow plugs are fully heated, or when the engine starts. Note that when the engine coolant temperature is above a minimum threshold, the glow plug heating system is deactivated automatically and the warning light will not illuminate when the ignition is turned on.

Nonetheless, depending on the nature of the failure, defect, or malfunction, the glow plug warning light may either not illuminate, or remain illuminated. In the former case, the glow plugs will generally not be heated, which could result in extended cranking times, no start conditions, and damage to both the starter motor and/or the battery. In the latter case, the glow plug heating circuits may not be deactivated, which usually results in the destruction of the glow plugs, and sometimes, in engine damage because the hot glow plugs ignite the air/fuel mixture prematurely.

To prevent the situations described above, the PCM will set code P0381 when it recognizes that it cannot control the glow plug heating circuits effectively, regardless of the nature of the failure.

Where is the P0381 sensor located?

The image above shows a simplified glow plug heating/control circuit, but note that while this circuit is not overly complicated, gaining access to some components or even much of the wiring for the purpose of testing resistance, continuity, or ground connections can be difficult on some applications. Non-professional mechanics should therefore always refer to the manual for the affected application to locate and identify all wiring and/or components in this circuit correctly, since a mistake could lead to a misdiagnosis and quite possibly the destruction of one or more components, which could include one or more control modules.

What are the common causes of code P0381 ?

Non-professional mechanics should take note that is extremely rare (if not totally unheard of) for all the glow plugs on an engine to fail at the same time. Therefore, if no glow plugs are working, the fault is almost certainly in the wiring, or in a component such as a relay or fuse.

Nonetheless, some common causes of code P0381 could include the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors anywhere in the glow plug heating circuit
• Defective engine coolant temperature sensor(s)
• Poor ground connections between the battery and the engine, and/or between the battery and the vehicle body
• Blown glow plug warning light bulb
• Defective glow plug relay or blown fuse/fusible link
• Defective glow plug heating timer and/or relay on older applications
• Defective ignition switch
• Defective glow plug control module

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