P0686 — ECM/PCM Power Relay Control Circuit Low

OBD-II trouble code P0686 indicates an issue with the control circuit of the engine control module (ECM) or powertrain control module (PCM) relay. In simple terms, the vehicle’s onboard computer is detecting that the circuit responsible for powering the ECM/PCM is experiencing an abnormal low voltage or electrical disturbance. This code can affect various vehicle functions and, if left unresolved, may lead to further drivability issues.

Note that labels for this code may vary between different vehicle manufacturers, and the exact terminology used by the OEMs might differ slightly. However, the underlying electrical issue remains consistent across brands.

Fast Facts:

System affected: Engine Control Module (ECM) or Powertrain Control Module (PCM) power relay control circuit

Severity: Moderate to High, depending on symptoms

Main symptoms: Engine stalling, no-start condition, loss of power

Driveability: Limited — the vehicle may run poorly or not at all until fixed

—

Manufacturer variations for trouble code P0686

The diagnostic trouble code P0686 can be labeled differently depending on the vehicle manufacturer and sometimes even the specific model or year. OEMs often have their unique terminology or code designations for similar issues, which can include variations in the sequence or wording used in the diagnostic software. It’s essential to confirm the exact label within the manufacturer’s service information system, but generally, the core issue remains the same: a problem with the ECM/PCM power relay control circuit.

Ford: P0686 — « Control Circuit (Passenger’s Side) of PCM Power Relay » (or similar variations)
Volkswagen: P0686 — « Control Circuit for PCM Power Relay » (may vary slightly in labels)
BMW: P0686 — « Engine Control Module Relay Control Circuit Low »
Mercedes-Benz: P0686 — « Control Circuit of Power Module, Too Low »
General example: The labels may include brief descriptions like « ECM/PCM Relay Control Circuit Low » or similar phrasing indicating an electrical control issue.

—

What does trouble code P0686 mean?

This code signifies that the vehicle’s ECU (Engine Control Unit) or PCM is detecting a problem in the control circuit of the relay responsible for supplying power to the module itself. The ECU/PCM relies on a relay—much like a digital switch—to manage high-current circuits. When this relay’s control circuit conditions fall below a specific voltage threshold or exhibit irregularities, the ECU flags this as a fault.

During normal operation, the ECU sends a control signal to the relay to allow or cut power to vital engine systems. The trigger for P0686 is usually an electrical anomaly—such as a voltage drop or poor connectivity—that prevents proper control of the relay circuit.

This code typically appears when a problem occurs during startup, after a jump-start, or following electrical system modifications or faults. It’s also frequently detected in vehicles aged or with wiring that has suffered wear or corrosion.

—

Severity and risks of trouble code P0686

This fault can lead to various engine and vehicle performance issues, including stalling, failure to start, or inconsistent engine operation. The risk level largely depends on whether the relay controls critical engine functions, such as fuel injection, ignition, or transmission control systems.

While it may not immediately threaten safety in all cases, ignoring this fault can result in more severe damage over time or immobilise the vehicle. The vehicle’s electronics could malfunction, leading to unpredictable behaviour, especially during driving conditions requiring precise control.

Ultimately, can the vehicle be driven? This depends on the severity and specific systems affected. In many cases, the vehicle can be driven with caution for a limited distance, but repair is highly recommended—especially if multiple symptoms are present.

—

Symptoms of trouble code P0686

Recognising the symptoms associated with P0686 can help in diagnosing the issue promptly. The symptoms often relate to electrical or engine control failures and may include:

Engine stalling or cutting out unexpectedly, especially during acceleration or idling
Difficulty starting the engine or a no-start condition
Intermittent engine performance, with occasional loss of power
Illuminated check engine or warning light on the dashboard
Erratic gauges or electrical malfunctions, such as dimming lights or ABS system issues
Failure of some electrical circuits controlled by the PCM/ECU

—

Most likely causes of trouble code P0686

Various factors can trigger the P0686 code, but some causes are more common than others. The underlying theme in most cases revolves around electrical integrity, relay functionality, and wiring health.

Faulty or failing PCM/ECM relay: The relay itself may be defective or worn out, leading to unreliable control signals.
Corroded, damaged, or loose wiring connections in the relay control circuit: Poor contact can cause voltage drops or intermittent signals.

3. Blown or damaged fuse: The fuse protecting the relay circuit could be blown, cutting power supply or control signals.

4. Voltage supply issues: Insufficient voltage or irregular power supply to the PCM or relay circuit can trigger this fault.

5. Faulty ECU/PCM: An internally damaged PCM or one with an internal control circuit malfunction may produce this code.

6. Electrical environmental factors such as water ingress or wiring insulation degradation.

—

How to diagnose trouble code P0686

Diagnosing P0686 requires a methodical approach to confirm electrical and wiring integrity. Here are the general steps for a safe, effective diagnosis:

Visual Inspection: Begin with a thorough visual check of the PCM relay, fuses, and wiring harnesses for signs of corrosion, physical damage, or loose connections.
Check the fuse: Verify that the fuse linked to the relay control circuit is intact and properly seated.
Test relay function: Remove the relay and test it using a relay tester or swap it with a known-good relay if compatible.
Inspect wiring connections: Ensure all wiring connectors are clean, secure, and free of corrosion or damage.
Measure voltage: Use a multimeter to confirm that the control circuit is receiving proper voltage during key-on and key-off states.
Check live data: Connect a scanner capable of reading live data to observe the relay control circuit’s status in real-time when turning the ignition on/off.
Continuity testing: Test the wiring between the relay control circuit and the PCM to ensure circuit continuity and absence of short circuits.
Further diagnostics: If all electrical checks are OK, consider testing the PCM output circuit or reprogramming the ECU if necessary, following manufacturer guidelines.

—

Possible repairs for trouble code P0686

Addressing the P0686 fault involves fixing the root causes identified during diagnostics. Basic repairs can often be performed by a knowledgeable DIY enthusiast, while others require professional intervention.

Replacing the PCM/ECU relay: A simple fix involving removing and installing a new relay—generally a repair for qualified technicians if the relay is integrated or specialized.
Repairing or replacing wiring and connectors: Fixing corrosion, repairing broken wires, or replacing damaged connectors are common solutions and usually straightforward.
Replacing blown fuses: Ensuring the fuse is correct and replacing it if necessary; this is a basic repair.
Restoring power supply: Addressing battery or alternator issues to ensure adequate voltage.
Reprogramming or replacing the ECU/PCM: If the internal control circuit of the PCM is faulty, a professional may need to reprogram or replace the unit.
Electrical system check-up: Diagnosing and fixing sources of wiring degradation or environmental damage.

—

Vehicles commonly associated with trouble code P0686 in Europe

The P0686 diagnostic trouble code is found across various vehicle makes and models, especially those with complex electrical systems. While specific data is limited, certain brands are notably affected due to their electrical architecture.

Most commonly, manufacturers like Ford, Volkswagen, BMW, Mercedes-Benz, and others have reported instances of this fault. It is often associated with engine models that employ sophisticated PCM/ECU control and relay systems. Typical engine families include various gasoline and diesel powertrains where electrical control circuits are critical for engine management and starting systems.

—

Frequent mistakes with trouble code P0686

Diagnosing and repairing P0686 can encounter pitfalls that prolong the repair process or lead to incorrect fixes. Being aware of common mistakes helps ensure a more efficient diagnosis:

Assuming the relay is always at fault without verifying its operation or checking electrical connections first.
Overlooking fuse issues or misidentifying the fuse related to the relay control circuit.
Missing visual inspection for wiring damage, corrosion, or loose connectors.
Ignoring underlying power supply issues such as battery health or alternator output problems.
Performing unnecessary ECU replacements without confirming signal continuity or circuit integrity.

—

Quick FAQ — trouble code P0686

Can this code disappear on its own?

It is unlikely that P0686 will resolve itself without intervention. Electrical issues typically persist until the faulty component or wiring is repaired or replaced.

Can I keep driving?

If the vehicle operates normally and there are no warning lights or drivability issues, short-term driving might be acceptable. However, persistent symptoms or warning lights suggest that repairs should be addressed promptly to avoid further damage.

Why does the code return after clearing?

The fault is caused by an electrical or circuit problem. If the underlying issue remains uncorrected, the code will reappear after clearing. Proper diagnosis and repair are crucial to prevent recurrence.