For diesel vehicle owners and automotive technicians, encountering a trouble code like P0675 can be a common yet crucial diagnostic event. At keyfobprog.com, we delve into the intricacies of automotive systems to provide you with expert knowledge. Today, we’re focusing on the P0675 code, which typically indicates an issue within the glow plug system, specifically pointing to a problem with cylinder 5’s glow plug circuit. Understanding the operation of glow plugs is paramount in accurately diagnosing and resolving this, and related, fault codes.
The Vital Role of Glow Plugs: Pre-Glow, Glow Stage, and After-Glow
Glow plugs are essential components in diesel engines, particularly in cold-start conditions. Unlike gasoline engines, diesel engines rely on compression ignition, where air is compressed to such a high degree that the temperature rises sufficiently to ignite the injected fuel. However, in cold weather, the engine block and combustion chamber absorb heat, making ignition more challenging. This is where glow plugs step in.
Pre-Glow Phase
When you turn your key to the “On” position, the engine control module (ECM) immediately assesses the engine coolant temperature. Based on this reading, the ECM calculates the necessary pre-heating time. It then activates the glow plug output stage and the indicator lamp on your dashboard. This output stage essentially switches on the current flow to the glow plugs. The pre-glow phase is crucial for heating the combustion chambers before engine cranking, ensuring reliable cold starts. Once the pre-glow period, determined by the ECM, is complete, the indicator lamp typically turns off, signaling that the engine is ready for starting. Critically, any malfunctions within this pre-glow system, such as faulty components or wiring issues, will be detected and often indicated by the glow plug lamp illuminating, and a diagnostic trouble code, like P0675, will be stored in the ECM’s memory.
Glow Plug Output Stage Operation
The glow plug output stage acts as the intermediary between the ECM and the glow plugs themselves. Upon receiving the “Ignition On” signal from the ECM, the output stage begins its operation. A key safety feature is built-in: if communication with the ECM is lost for any reason, the glow plug stage will automatically terminate after a brief two-second interval. This output stage is also intelligent, constantly monitoring its operational status (ON/OFF) and diligently detecting system faults. Crucially, it transmits any detected faults back to the ECM. These recognized faults include:
- Open circuit in glow plug leads: This signifies a break in the wiring to one or more glow plugs, preventing current flow and thus heating.
- Short circuit in the glow plug circuit: A short circuit can lead to excessive current flow, potentially damaging the glow plugs or the control module.
- Output stage fault or temperature-related shut-off: The output stage itself can malfunction, or it may trigger a temperature-related shut-off to protect the system from overheating.
It’s important to note that if a temporary fault occurs in the glow plug system, the indicator lamp might illuminate only while the fault is actively present. If the issue becomes intermittent or resolves itself, the lamp may switch off, but the fault code, such as P0675 indicating a Cylinder 5 glow plug circuit issue, will likely remain stored in the ECM, providing valuable diagnostic information for technicians.
After-Glow Phase
The operation of glow plugs isn’t limited to just starting the engine. Many modern diesel systems employ an after-glow phase. Once the engine is running, the ECM continues to monitor the coolant temperature. Based on this, it determines an after-glow duration. During this phase, the glow plugs remain active, being controlled by the glow plug output stage. This after-glow functionality provides several benefits: smoother engine running immediately after a cold start, improved engine warm-up efficiency, reduction or elimination of blue exhaust smoke often associated with cold diesel starts, and a more stable idle speed during the initial cold running period. Interestingly, if the coolant temperature sensor signal is unavailable (due to failure, for example), the system is often designed to use the oil temperature sensor signal as a substitute, ensuring continued after-glow operation when needed.
Understanding the Glow Plug Control Module
The Glow Plug Module (sometimes referred to as the Glow Plug Control Module or Glow Plug Relay Module) is the central control unit for the entire glow plug system. It’s directly connected to the vehicle’s battery positive (B+) circuit via a fuse and is always grounded through the engine wiring harness. This constant power and ground supply allows the module to be operational regardless of the ignition switch position.
When the ECM receives the “Ignition On” and “Ignition Run” signals from the electronic ignition switch (EIS), it communicates with the Glow Plug Module via a LIN (Local Interconnect Network) bus. This communication instructs the module to actuate the glow plugs for a specific duration, precisely calculated by the ECM based on ambient air and engine coolant temperatures.
Inside the Glow Plug Module, integrated relays and microprocessors manage the power delivery to the individual glow plugs according to the ECM’s commands. Crucially, these modules are equipped with on-board diagnostic capabilities. They can detect faults within the glow plug circuits and generate powertrain diagnostic trouble codes (DTCs). These DTCs, including codes like P0675, are then transmitted back to the ECM via the LIN bus.
Because of the sophisticated microprocessor-controlled voltage regulation within these modules, traditional diagnostic methods, such as simple voltage checks, might not always be reliable when troubleshooting glow plug system problems. Accurate diagnosis often necessitates the use of a diagnostic scan tool capable of reading LIN bus data and interpreting the electronic communication related to glow plug module operation. This is essential for correctly identifying issues and resolving codes like P0675, ensuring the optimal performance and longevity of your diesel engine. When facing a P0675 code, remember to consider the entire glow plug system, from the plugs themselves to the module and its control circuitry, for a comprehensive and effective diagnostic approach.
