Description of the P0688 fault code.
OBD2 Error Codes

P0688 Engine/Transmission Control Module (ECM/PCM) Power Relay Sensor Circuit Open/Failure

26.05.2022 /

P0688 – OBD-II Trouble Code Technical Description

Trouble code P0688 is a generic trouble code that indicates a malfunction of the Engine Control Module (ECM) or Powertrain Control Module (PCM) power relay control circuit.

What does trouble code P0688 mean?

Trouble code P0688 indicates a problem in the engine control module (ECM) or powertrain control module (PCM) power relay control circuit in the vehicle. This code occurs when the ECM/PCM power relay control circuit does not provide normal voltage as specified by the manufacturer's specifications.

The ECM and PCM are vehicle components responsible for controlling the engine and other vehicle systems. They receive power through a relay that turns power on or off from the battery. The P0688 code indicates that there is a problem with this power circuit, which can cause the engine or other vehicle systems to not operate properly. It is important to note that this code typically only appears on vehicles that use the ECM/PCM power relay and may not apply to other types of vehicles or engine management systems.

Fault code P0688.

Possible reasons


Possible reasons for DTC P0688:

  • Damaged or broken wires: The wires connecting the power relay to the ECM/PCM or to the power supply may be damaged, broken, or burned, resulting in loss of electrical contact and insufficient power.
  • Poor connections or oxidation of contacts: It is necessary to check the condition of the connections and contacts in the power relay control circuit. Oxidation or poor connections can lead to reduced electrical contact and resulting in insufficient power supply.
  • Faulty power relay: The power relay itself may be defective, resulting in insufficient power transfer to the ECM/PCM.
  • Battery problems: Low voltage or improper battery operation may cause insufficient power to the ECM/PCM through the power relay.
  • Grounding problems: Insufficient or improper grounding in the circuit can also cause the power relay to malfunction and the ECM/PCM to have insufficient power.
  • Problems with the ignition switch: If the signal from the ignition switch does not reach the power relay, it may result in insufficient power to the ECM/PCM.
  • ECM/PCM malfunction: In rare cases, the ECM or PCM itself may be defective, resulting in insufficient power or other problems with the control system.

It is important to conduct a thorough diagnosis to determine the exact cause of the P0688 code before performing repair actions.

What are the symptoms of a fault code? P0688?

If DTC P0688 is present, you may experience the following symptoms:

  • Problems with starting the engine: Low voltage on the power relay control circuit may cause the engine to be difficult or unable to start.
  • Loss of power: Insufficient power to the ECM or PCM may cause loss of engine power or unstable operation.
  • Unstable engine operation: Improper power supply may cause the engine to run erratically, such as shaking, shaking or jerking when driving.
  • Limitation of vehicle functions: Some vehicle functions that depend on the ECM or PCM may not operate properly or be unavailable due to insufficient power.
  • Check Engine Light Appears: Code P0688 activates the Check Engine light on the dashboard, indicating problems with the electrical system.
  • Loss of electrical components: Some vehicle electrical components, such as lights, heaters, or climate controls, may work less efficiently or fail altogether due to insufficient power.
  • Speed ​​Limit: In rare cases, the vehicle may go into limited speed mode due to electrical system problems caused by code P0688.

If you experience these symptoms on your vehicle and have DTC P0688, it is recommended that you have the problem diagnosed and repaired by a qualified automotive technician.

How to diagnose a fault code P0688?

Diagnosing the P0688 trouble code involves several steps to identify and resolve the problem, the basic steps to follow when diagnosing this error are:

  1. Checking the battery: Make sure the battery voltage is within the normal range and that it is charged. Check the condition of the terminals and wires on the battery for corrosion or poor connections.
  2. Checking wiring and connections: Inspect the wires from the power relay to the ECM/PCM for damage, breaks, or burns. Also check connections and contacts for oxidation or poor contact.
  3. Checking the power relay: Check the power relay itself for functionality. Make sure it functions correctly and provides stable power to the ECM/PCM.
  4. Grounding check: Verify that the ground on the power relay control circuit is operating correctly and provides a reliable ground for system operation.
  5. Checking the signal from the ignition switch: Check if the signal from the ignition switch reaches the power relay. If necessary, check the condition and functionality of the ignition switch itself.
  6. Using the Diagnostic Scanner: Connect a diagnostic scan tool to the OBD-II port and read trouble codes to get more information about the problem and system status.
  7. Additional tests: Perform additional tests such as voltage testing at various points in the control circuit and additional electrical component checks if necessary.

After diagnosing and identifying the possible cause of the P0688 code, you can begin to resolve the problem by repairing or replacing the faulty components. It is important to carry out diagnostics carefully and systematically to avoid mistakes and correctly determine the cause of the problem. If you do not have experience in diagnosing and repairing vehicles, it is recommended that you contact a professional auto mechanic or auto repair shop for assistance.

Diagnostic errors

When diagnosing DTC P0688, the following errors may occur:

  • Insufficient battery check: Some technicians may skip checking the battery condition or not considering its effect on the voltage in the power relay control circuit.
  • Unreasonable replacement of the power relay: Instead of a thorough diagnosis, they may immediately replace the power relay, which may be unnecessary if the problem lies in another component.
  • Ignoring other problems with the electrical system: Trouble code P0688 can be caused by various factors such as damaged wires, poor connections, or problems with the ignition switch. Ignoring these factors may lead to incorrect diagnostic conclusions.
  • Misunderstanding of technical specifications: Not all technicians can correctly interpret manufacturer specifications, which can lead to incorrect diagnoses and repair actions.
  • Insufficient ground and input checks: Grounding problems or incorrect input signals can also cause P0688 but may be missed during diagnosis.
  • Faulty diagnostic tools: Using faulty or uncalibrated diagnostic tools may result in incorrect diagnostic results.
  • Insufficient experience and knowledge: Insufficient experience or knowledge of vehicle electrical systems may lead to incorrect diagnoses and repair actions.

To successfully diagnose the P0688 trouble code, it is important to perform all necessary tests and analyze all available information to determine the correct cause of the problem.

How serious is the fault code? P0688?

Trouble code P0688 is quite serious because it indicates a problem in the engine control module (ECM) or powertrain control module (PCM) power relay control circuit in the vehicle. If the voltage in this circuit is not normal, it can result in insufficient or unstable power supply to the engine control systems, which in turn can cause a number of serious problems, including:

  • Problems with starting the engine: Low voltage or malfunctioning power relay may make starting the engine difficult or impossible.
  • Loss of power and unstable engine operation: Insufficient power supply to the ECM/PCM can result in loss of engine power, rough operation, or even cylinder misfire, which can significantly reduce vehicle performance.
  • Limitation of vehicle functions: Some vehicle functions that depend on the ECM or PCM may not operate properly or be unavailable due to insufficient power supply.
  • Risk of damage to other components: Incorrect power supply may cause overheating or damage to other electrical system components or even damage to the ECM/PCM.

Due to the above consequences, code P0688 requires serious attention and immediate correction of the problem. Diagnosis and repairs must be made as soon as possible to avoid further problems and ensure safe and reliable operation of the vehicle. If you experience code P0688, it is recommended that you take it to a qualified automotive technician for diagnosis and repair.

What repair will help eliminate the code? P0688?

Resolving trouble code P0688 requires a series of diagnostic steps to determine the exact cause of the problem. Depending on the identified cause, the following repair actions may be required:

  1. Replacing or repairing damaged wires and connections: If damaged or broken wires are found, they should be replaced or repaired. It is also necessary to ensure reliable connections and eliminate contact oxidation.
  2. Replacing the power relay: If the power relay is faulty, you need to replace it with a new one that is compatible with your vehicle.
  3. Improved grounding: Check and improve the grounding in the power relay control circuit, making sure the contacts are clean and reliable.
  4. Checking and repairing the ignition switch: Check the condition and functionality of the ignition switch. Replace or repair the switch if necessary.
  5. Battery check and maintenance: Make sure the battery is charged and working properly. Replace it or carry out maintenance if necessary.
  6. Check and, if necessary, replace the ECM/PCM: In rare cases, the problem may be due to a problem with the control module itself. In this case, the ECM/PCM may need to be replaced or repaired.
  7. Additional diagnostic and repair work: Perform additional tests and diagnostics to ensure that all system components are operating correctly. Perform additional repairs if necessary.

It is important to correctly identify the cause of the P0688 problem before carrying out repair work. If you do not have the experience or skills to perform the repair yourself, it is recommended that you contact a qualified automotive technician or service center for diagnosis and repair.

P0688 error code explained and solution

P0688 – Brand-specific information

List of some car brands with decoding of trouble code P0688:

This is just a small list of vehicle brands that may experience trouble code P0688. It is important to note that the causes and solutions to this problem may vary slightly depending on the specific model and year of the vehicle.

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