What does it 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 100 0 F. 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.