P0686 — ECM/PCM Power Relay Control Circuit Low #

The definition of this code may vary by manufacturer.
Known description variations:

• “ECM/PCM Power Relay Control Circuit Low” (General, American, Asian brands)
• “Main Relay Control Circuit Low Voltage” (Honda/Acura)
• “ECU Power Hold Relay Control Circuit Low” (Mazda/Ford)

Summary of DTC P0686 #

DTC P0686 refers to a detected low voltage condition in the ECM/PCM Power Relay Control Circuit. The ECM (Engine Control Module) or PCM (Powertrain Control Module) is the vehicle’s main computer. The power relay is an electronic switch that supplies the ECM/PCM with electrical power when the ignition is on. If the circuit controlling this relay shows a lower-than-expected voltage, the module may not function correctly or at all. This code can lead to engine performance problems, starting issues, or even a no-start condition. Fast, accurate diagnosis is key to avoid further electrical issues or unexpected breakdowns.

Common Symptoms for DTC P0686 #

Issues with the ECM/PCM relay circuit often present themselves quickly. Most drivers will notice changes in how the car starts or runs. Sometimes, the symptoms are subtle until the relay or circuit fails completely. Watch for these signs:

• Engine cranks but won’t start
• Intermittent stalling
• Engine cuts out unexpectedly
• Check Engine Light (CEL) illuminated
• Reduced engine performance
• Electrical accessories may not function properly

Probable Causes for DTC P0686 #

This code is most often triggered by issues in the relay circuit itself, but other components can also contribute. Here are the most common culprits, listed by likelihood:

1. Faulty ECM/PCM power relay: The relay may have failed internally or have worn contacts.
2. Open, shorted, or corroded wiring in the relay control circuit: Wire damage or poor connections can lower voltage.
3. Poor ground connection for the ECM/PCM or relay: Corrosion or looseness can disrupt the circuit.
4. Faulty ECM/PCM: Rare, but possible if the module itself cannot complete the control circuit.
5. Blown fuse feeding relay or ECM/PCM power circuit.
6. Aftermarket accessory interference: Poorly installed alarms or remote starts may disrupt the circuit.

Step-by-Step Diagnostic Guide for P0686 #

Diagnosing P0686 requires methodical checks with a digital multimeter and sometimes a scan tool. Always start with the basics and work towards more complex possibilities. Here’s a structured approach:

1. Check for additional codes: Other power or relay-related codes may provide clues.
2. Inspect fuses: Verify all ECM/PCM and relay-related fuses are intact.
3. Locate the ECM/PCM power relay: Consult your OEM (Original Equipment Manufacturer) wiring diagram.
4. Test relay function: Swap with a known good relay if possible, or bench test for continuity and operation.
5. Inspect wiring and connectors: Look for corrosion, broken wires, or loose connections at the relay, ECM/PCM, and fuse block.
6. Check circuit voltage: With the ignition on, measure voltage at the relay control circuit; compare to OEM specs.
7. Test ground connections: Use a multimeter to ensure low resistance at ground points.
8. Scan ECM/PCM: If all wiring and relay tests pass, check the module itself for internal faults.

Possible Repairs for P0686 #

Once the root cause is identified, repairs must target the specific failure. Here are common solutions matched to the causes above:

• Replace faulty ECM/PCM power relay
• Repair or replace damaged wiring/connectors (clean, solder, or use OEM harness repair kits)
• Clean or tighten ground connections for relay and ECM/PCM
• Replace blown fuse with correct OEM rating
• Remove or correct faulty aftermarket installations interfering with the circuit
• Replace ECM/PCM (only after exhaustive testing and confirmation)

Is It Safe to Drive with P0686? #

Generally, it is NOT recommended to drive with DTC P0686 active. This code can lead to sudden engine shutdown or a no-start condition, potentially leaving you stranded. If the engine is running, it may stall without warning, posing a safety risk. Immediate diagnosis and repair are advised to preserve vehicle reliability and safety.

Vehicles Most Commonly Affected by P0686 #

While P0686 is a generic code, it appears more frequently in certain models, especially those with complex relay and power distribution setups. Based on field experience, vehicles often affected include:

• Honda/Acura (Accord, Civic, CR-V, Pilot)
• Mazda (Mazda3, Mazda6, CX-5)
• Ford (Focus, Fusion, Escape)
• Chrysler/Dodge/Jeep (various models with TIPM modules)
• GM/Chevrolet (Malibu, Cruze, Equinox)
• Nissan (Altima, Sentra, Rogue)

Common Mistakes & Best Practices for P0686 #

Misdiagnosis is common when troubleshooting power relay codes. Avoid these pitfalls and follow best practices for a successful repair:

• Replacing the relay without checking wiring and grounds: Always inspect the entire circuit before part replacement.
• Skipping fuse and power supply checks: A simple blown fuse can mimic more serious issues.
• Overlooking corrosion or loose connections: Weather and age can degrade contacts and grounds.
• Assuming the ECM/PCM is faulty: Module replacement should be a last resort after all other options are exhausted.
• Not using OEM wiring diagrams: Always consult up-to-date technical resources for your exact model.

Express FAQ: DTC P0686 #

Here are quick answers to common questions about this trouble code:

• Q: Can a weak battery cause P0686?
  A: Rarely. While a weak battery can cause many electrical issues, P0686 specifically points to the relay control circuit. However, always verify proper system voltage as part of your diagnosis.
• Q: Will clearing the code fix the problem?
  A: Clearing the code without addressing the root cause will not resolve the issue. The code will return if the circuit fault remains.
• Q: Can I bypass the relay?
  A: Bypassing the relay is not recommended and may damage your ECM/PCM or cause further electrical problems. Always repair according to OEM standards.