Description of the P0684 fault code.
OBD2 Error Codes

P0684 Circuit Range/Performance Between Glow Plug Control Module and PCM

26.05.2022 /

P0684 – OBD-II Trouble Code Technical Description

Trouble code P0684 is a general trouble code that indicates there is a problem with the glow plug control module and its communication with the vehicle's PCM.

What does the fault code mean P0684?

Trouble code P0684 indicates possible communication problems between the powertrain control module (PCM) and the glow plug control module. This means that there is a problem communicating or sending commands between the two modules.

Typically, glow plugs are used in diesel engines to preheat the air in the cylinders before starting the engine, especially in cold conditions. The glow plug control module controls this process. The P0684 code may indicate a faulty wiring between the PCM and the glow plug control module or a faulty glow plug control module itself. This can lead to difficulty starting the engine, especially in cold weather, and other engine performance problems.

Fault code P0684.

Possible reasons

Several possible causes for trouble code P0684:

  • Damaged wiring: Damage or breaks in the electrical wiring between the PCM and the glow plug control module may result in incorrect transmission of data or commands.
  • Malfunction of the glow plug control module: The glow plug control module itself may become damaged or fail, causing improper communication with the PCM.
  • Problems with PCM: Faults or errors in the PCM can also be the cause of the P0684 code as it is the central control unit in the vehicle.
  • Corrosion or oxidation of contacts: Corrosion or oxidation of the contacts on the connectors or connections between the PCM and the glow plug control module can cause poor contact and incorrect data transmission.
  • Problems with the electrical system: General problems with the vehicle's electrical system, such as insufficient voltage or shorts, can also cause the P0684 code.
  • Problems in other systems: Malfunctions in other vehicle systems, such as the ignition system or fuel injection system, can also cause P0684 by affecting PCM operation.

To accurately determine the cause of the P0684 code, it is recommended to conduct a thorough diagnosis of the vehicle.

What are the symptoms of a fault code? P0684?

Symptoms for DTC P0684 may vary depending on the specific cause and context of the problem. Some common symptoms that may occur with this error are:

  • Difficulty starting the engine: One of the most common symptoms of P0684 is difficulty starting the engine, especially in cold weather. This may occur due to improper operation of the cylinder preheating system or improper management of the glow plugs.
  • Unstable engine operation: The engine may experience rough operation at idle or while driving, including shaking, rattling, or uneven power.
  • Power limitation: The engine management system may put the engine into a limited power mode to prevent further problems or damage if it detects the P0684 code.
  • Error messages appearing on the dashboard: Error indicators may appear on the instrument panel, indicating problems with the engine management system or electrical circuit.
  • Loss of efficiency: Increased fuel consumption or decreased overall engine performance may occur due to improper control of the glow plugs or other control system components.
  • Glow plugs not working: In some cases, if the problem is with the glow plug control module, the glow plugs may stop functioning, causing the engine to perform poorly when starting.

If you notice any of these symptoms or if the P0684 code appears, it is recommended that you contact a qualified auto mechanic or service center for diagnosis and repair.

How to diagnose a fault code P0684?

To diagnose DTC P0684, follow these steps:

  1. Scan error code: Use a diagnostic scanner to read error codes from the engine management system. Make sure the P0684 code is present and not a false positive.
  2. Visual inspection: Inspect the electrical wiring and connections between the powertrain control module (PCM) and the glow plug control module for damage, corrosion, or breaks.
  3. Electrical circuit check: Use a multimeter to check the voltage and resistance in the electrical circuit between the PCM and the glow plug control module. Make sure the wires and connections are intact and functioning correctly.
  4. Checking the glow plug control module: Check the glow plug control module for damage or malfunction. If there is any doubt about the operation of the module, it may need to be tested or replaced.
  5. Check PCM: Check the operation of the PCM and its communication with the glow plug control module. Make sure the PCM is receiving the correct signals from other sensors and is sending the correct commands to the glow plug control module.
  6. Additional checks: Check the condition of other components of the ignition and fuel injection system, such as temperature and pressure sensors, which may affect the operation of the glow plugs.
  7. Road testing: After performing all necessary diagnostic procedures, test run the engine and perform a road test to ensure the problem is resolved.

Remember that accurately diagnosing the P0684 code may require specialized equipment and knowledge, so if in doubt or lack of experience, it is best to contact a qualified auto mechanic or service center.

Diagnostic errors

When diagnosing DTC P0684, the following errors may occur:

  • Skipping a visual inspection: Insufficient attention to visual inspection of electrical wiring and connections may result in obvious problems such as damage or breaks being missed.
  • Misinterpretation of test results: Incorrect interpretation of electrical circuit or glow plug control module test results may lead to erroneous conclusions about the cause of the malfunction.
  • Insufficient diagnostics of other components: Skipping diagnostics on other components, such as the PCM or sensors that may affect glow plug performance, may result in a failed repair.
  • Incorrect priority of repair actions: Deciding to start a repair by replacing the glow plug control module without performing a full diagnostic may result in wasting time and resources on unnecessary repair work.
  • Not taking into account the influence of surrounding factors: Certain factors, such as corrosion or oxidation, can affect the electrical circuit and cause P0684, but may be missed during diagnosis.
  • Incorrect interpretation of scanner data: Incorrect interpretation of data obtained from the diagnostic scanner may lead to incorrect diagnosis and incorrect repairs.

It is important to carry out a thorough and systematic diagnosis, taking into account all possible causes of the P0684 code and eliminating them one by one in order to avoid repair mistakes.

How serious is the fault code? P0684?

Trouble code P0684 should be taken seriously, especially considering its impact on the performance of the cylinder preheating system (in the case of diesel engines) and overall engine performance. Here are a few reasons why this error code requires serious attention:

  • Difficulty starting the engine: Malfunctions in the glow plug control system can lead to difficulty starting the engine, especially on cold days. This can be a problem, especially if the car is used for driving in cold temperatures.
  • Negative impact on performance: Improper operation of the glow plugs can affect engine performance, resulting in reduced power and operating efficiency.
  • Risk of engine damage: If the problem remains unresolved, it may cause additional damage to the engine or other system components.
  • Power limitation: To prevent further damage, the engine management system may place the engine in a power-limited mode, which may reduce overall vehicle performance.
  • Potential problems on the road: If the problem occurs while driving, it may create a dangerous situation on the road due to loss of power or improper operation of the engine.

So, trouble code P0684 is serious and needs to be addressed promptly to ensure the safety and reliable operation of the vehicle.

What repair will help eliminate the code? P0684?

Resolving trouble code P0684 requires diagnostics and possibly a number of repair actions depending on the specific cause of the problem, some possible remedies are:

  1. Checking and restoring electrical wiring: Check the electrical wiring and connections between the powertrain control module (PCM) and the glow plug control module for damage, breaks, or corrosion. Repair or replace damaged wiring sections.
  2. Replacing the glow plug control module: If diagnostics indicate a faulty glow plug control module, replace it with a new or working one.
  3. Overhaul or replace the PCM: If problems are found with the PCM, the unit may need to be repaired or replaced.
  4. Cleaning and updating connections: Clean and update the contacts and connectors between the PCM and the glow plug control module to ensure reliable communication.
  5. Checking and replacing sensors: Check the operation of sensors such as temperature and pressure sensors that may affect the glow plug control system. Replace faulty sensors if necessary.
  6. Updating the software: Perform a PCM software update, if available, to resolve known errors or improve control system performance.
  7. Professional diagnostics and repair: In case of complex or unclear causes of the P0684 code, it is recommended that you contact a qualified auto mechanic or service center for professional diagnosis and repair.

The choice of specific repair action depends on the diagnostic results and the identified causes of the P0684 error.

How to Fix P0684 Engine Code in 3 Minutes [2 DIY Methods / Only $9.29]

P0684 – Brand-specific information

Trouble code P0684 can be applied to various makes of cars, decoding for several of them:

Each car manufacturer may use their own terms and definitions for trouble codes, but the general meaning remains approximately the same. It is important to consult your service manual or authorized dealer for more detailed information on your specific make and model of vehicle.

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