|Trouble Code||Fault Location||Probable Cause|
|P0674||Glow plug, cylinder 4 -circuit malfunction||Wiring, poor connection, relay, glow plug control module, glow plug, ECM|
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What Does Code P0674 Mean?
OBD II fault code P0674 is a generic code that is defined as “Cylinder 4 Glow Plug Circuit/Open”, and is set when the PCM (Powertrain Control Module) detects an electrical defect/failure/malfunction that causes a break in continuity in the control circuitry of the glow plug on cylinder #4. Note though that on most applications, this code will also set when the PCM detects a variation of 10% or more in the required control system voltage. Note that this code applies only to applications with compression ignition (diesel) engines.
Although diesel engines rely on compression alone to ignite the air/fuel mixture, ignition of the mixture is very difficult to accomplish when the engine is cold. Therefore, all diesel engines are fitted with glow plugs whose tips extend into the pre-combustion chamber, and whose purpose it is to assist in the initial heating of the air/fuel mixture when the engine is cold.
In terms of operation, a glow plug is a metal cylinder that contains a heating element that is in its turn, embedded in a ceramic material that transports the elements’ heat to the outside metal wall of the glow plug through convection. When a current is applied to the glow plug, the element heats the plug’s tip to a very high temperature, which serves as a heat source to initiate ignition of the air/fuel mixture in cold engines.
On older systems, the glow plugs were controlled by a simple relay combined with a timer that controlled the glow plugs’ duty cycle, or “on” time. On these systems, power was fed to the timer/relay when the ignition is turned to the “ON” position, and while these systems worked reasonably well, their biggest drawback was that it could take ten seconds or more to heat the glow plugs sufficiently for the engine to start. Moreover, these systems’ “ON” time was limited to the amount of time the timer was set to, and if the engine did not start on the first attempt, the cycle had to be repeated.
However, on modern systems, the glow plugs are controlled by a dedicated glow plug control module that has the ability to sense the electrical resistance of each individual glow plug. The practical advantages of this is that the control module can modulate the current fed to each individual glow plug, which means that all glow plugs are heated to the same temperature in the same amount of time. Consider the image below-
This image shows the progression of heat through a single glow plug from cold, to fully heated. While this process on older systems could take ten seconds or more, improvements in glow plug design coupled with electronic control systems means that all the glow plugs on an engine can now be heated in less than two seconds. This not only reduces start-up times even in sub-zero temperatures, but also extends the lives of the glow plugs and other components such as the starter motor.
Nonetheless, it must be noted that glow plugs are not required to start a hot or even warm engine. To prevent the glow plugs on both old and newer systems from being activated when the engine is hot, the PCM uses input data from a dedicated engine coolant temperature sensor. When the coolant temperature exceeds a predefined limit, which is usually about 1040F, the PCM deactivates the glow plug control system to prevent both pre-ignition of the air/fuel mixture, and damage to the glow plugs and/or engine.
Where is the P0674 sensor located?
The image above shows the location of the glow plugs on a VW Jetta engine. In this instance, the glow plugs are provided with individual leads (not shown here) from the glow plug control module, and at first glance, the leads and the glow plugs themselves look almost like miniature spark plugs.
On older applications, the glow plugs are often connected in series with a bus bar. On these applications, the glow plugs can be located simply by looking for a length of flat steel or thick wire that connects all the glow plugs together. On V-type engines, each bank of cylinders will have a separate bus bar that connects all the glow plugs together on each bank of cylinders. Note though that bus bars may or may not be insulated.
What are the common causes of code P0674 ?
It should be noted that since glow plugs from a part of their control circuits, the failure of the glow plug itself on cylinder #4 would also set code P0674.
Some other common causes of code P0674 could include the following-
- Defective glow plug
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the glow plug control circuit
- Defective glow plug timer and or glow plug relay on applications that use old-style timers and relays
- Defective glow plug control module
- Defective glow plug lead on applications that use glow plug control modules
- Failed or failing PCM on systems that use glow plug control modules, but since this a rare event the fault must be sought elsewhere before any control module is replaced