This code indicates that the engine computer has detected an open circuit in the glow plug system for cylinder 12. An open circuit means the electrical path is broken, preventing the glow plug from heating the combustion chamber, which is critical for starting a diesel engine in cold conditions.
The internal heating element of the glow plug burns out over time, breaking the electrical circuit and triggering the open circuit code.
Corroded, frayed, or disconnected wiring and terminals specific to the cylinder 12 glow plug circuit interrupt the electrical path.
A failed glow plug system fuse or relay can cut power to the circuit, though this often triggers codes for multiple cylinders rather than just one.
The module that switches power to the glow plugs can fail internally, resulting in a loss of output to the specific cylinder 12 circuit.
Rarely, an internal driver failure inside the ECM can prevent it from commanding the glow plug circuit, causing an open circuit code.
You can usually drive the vehicle once it is running, but you will experience hard starts in cold weather. Prolonged cranking can wear out the starter and drain the battery.
Rarely. Drivers are sometimes told to replace the PCM for glow plug codes, but this is almost always incorrect. The actual cause is typically a burned-out glow plug or damaged wiring, and the PCM should only be considered as a last resort after all other components are verified.
It is highly recommended to replace all glow plugs at the same time. They wear at similar rates, so if one has failed, the others are likely close behind, and accessing them often requires removing the same engine covers.
Ignoring the code can lead to an inability to start the engine in cold weather, increased engine wear from unburned fuel washing oil from the cylinder walls, and potential damage to the starter motor from excessive cranking.