Comprehensive diagnostic guide for OBD-II code P0686

ECM/PCM Power Relay Control Circuit Low

Summary

• Definition (from open-source code definition): P0686 corresponds to a low signal in the ECM/PCM/TCM power relay control circuit. In practice, this means thePCM/ECM is not providing the expected control signal to the power relay that feeds the ECM/PCM itself (or the relay coil is not being driven properly) .
• Category: OBD-II Powertrain/Powertrain Code (P0 family) - a generic powertrain diagnostic code reported by the engine control module (ECM/PCM) .
• Core implication: The ECM/PCM is not properly energizing or is failing to command the power supply relay, which can prevent the ECM/PCM from receiving or maintaining power, leading to no-starts, stalling, or loss of engine management functions.

Step-by-Step Diagnosis

1) Confirm and frame the problem

• Retrieve the code with a scan tool and note any related DTCs (especially other PCM/communication or power-related codes).
• Confirm that P0686 is the only active code or part of a broader set of power-supply/ground issues. Wikipedia describes DTCs and Powertrain Codes as ECM/PCM-involved conditions that can appear with related faults .

2) Basic power/wire integrity checks

• Battery and charging: verify battery voltage is healthy (ideally >12.6 V at rest; maintain voltage under cranking). A weak battery can fail to energize the relay or produce a low-control signal.
• Main fuses and fusible links: inspect ECM power fuse(s) and main power circuits feeding the ECM relay bank; any blown fuse or corroded fusible link can cause a perceived "low relay control" condition.
• Grounds: verify the ECM/PCM ground is solid and free of corrosion or high resistance connections. A poor ground can cause the PCM to fail to drive the relay circuit properly.

3) Inspect the ECM power relay and power-relay circuit

• Locate the ECM power relay in the power distribution/relay box.
• Visually inspect the relay and sockets for corrosion, heat damage, bent pins, or melted plastic.
• Verify the input (B+ / battery feed) to the relay coil circuit is present when ignition is ON.
• Verify the relay coil control signal from the PCM:
  • If the PCM is supposed to energize the relay coil, measure the coil-control wire (often labeled Terminal 85 or 86 on the harness). Confirm whether the PCM provides a drive signal (voltage or ground) when ignition is ON.
  • If the coil energizes properly (you should hear/feel a click and see coil resistance in the expected range on the bench), the problem is likely not the relay coil itself.
• If the relay does not energize:
  • Confirm there is voltage on the coil input side when ignition is ON (or the other side is properly grounded, depending on design).
  • Confirm the PCM is sending the correct control signal (check PCM pin, measure continuity between PCM control wire and relay coil).
  • If there is no control signal and wiring checks are clean, suspect PCM output fault or PCM power supply fault.

4) Verify ECM power supply circuit (the "30" or main supply to the ECM)

• With ignition ON, measure voltage at the ECM power input pin (the supply to the ECM). It should be near battery voltage if the relay is energized and the ECM is powered.
• If there is battery voltage at the relay input but not at the ECM, inspect the wiring between the relay output circuit and the ECM power pin (check for broken wires, high-resistance connections, or corrosion).
• If there is no battery voltage at the ECM power input, the problem may be upstream (fuse, relay, or wiring).

5) Check the ECM/PCM control circuit integrity

• Check for continuity and resistance in the PCM control wire(s) going to the relay coil. Look for shorts to ground or battery, or broken conductors.
• Use a scope or a high-quality DVOM to observe the control signal while the ignition is ON and during cranking. A healthy PCM output should show a defined control waveform (a digital-type drive or a solid ground depending on design). If the signal is stuck at ground or floating, the PCM or its wiring may be failing.
• If the control signal looks correct but the relay does not energize or the ECM power does not reach the ECM, revisit the power supply path and relay function.

6) Check for PCM health indicators

• If the relay energizes correctly and ECM power is present, but P0686 persists, consider PCM internal failure or a fault in the ECM's relay-drive transistor.
• In rare cases, PCM memory or power-supply regulators can fail, causing misbehavior in relay control. If all external wiring and the relay test OK, PCM replacement may be warranted (per service information for the specific vehicle).

7) Recheck after repairs

• Clear the codes and perform a road test or simulated test drive to confirm P0686 does not reappear.
• Verify that the ECM powers up properly, and the relay is energizing in the expected ignition/engine state.

Common Causes

• Faulty or stuck ECM/PCM power relay (or its fuse/wiring): around 40%
• Wiring/conductors in the power-relay control circuit (damaged insulation, corrosion, poor connections, short to ground): around 25%
• ECM/PCM internal fault (drive transistor or regulator issues within PCM): around 15%
• Battery/charging or parasitic drain causing voltage drop or insufficient current to energize the relay: around 10%
• Other (shorts, high-resistance connections, or less common failures in the relay circuit): around 10%

Notes on these probabilities

Repair Options

• If a faulty relay is confirmed:
  • Replace the ECM power relay with a OEM or high-quality equivalent. Re-test to confirm the control signal energizes the relay and the ECM power is established.
• If a blown fuse or damaged fusible link is found:
  • Replace the fuse/fusible link and repair any associated wiring damage; re-test the system.
• If wiring or connector problems are found:
  • Repair or replace worn wiring, corroded connectors, or damaged harness segments. Re-seat and clean all connectors to ensure good contact.
• If the PCM control signal is absent or erratic:
  • Re-check the PCM control wire for shorts or opens; verify PCM ground integrity.
  • If all external wiring tests pass and the PCM control signal cannot be reproduced or the relay cannot be driven, consider PCM service, reprogramming, or replacement per vehicle-specific service information.
• If the ECM power input is missing voltage:
  • Trace back to the battery, fuse box, and relay input to locate an upstream fault; repair as needed.

Notes on design variations

• Some vehicles energize the relay coil by providing a supply to the coil and then switching ground via the PCM; other designs source ground to the coil and switch the supply via the PCM. The diagnostic approach remains the same: verify that the coil sees the correct supply and that the PCM provides the correct switch signal. Always verify the exact wiring diagram for the applicable vehicle to identify terminal designations and expected behavior.

Verification and testing plan after repair

• Clear DTCs and perform a test drive under typical operating conditions to ensure P0686 does not return.
• Recheck live data for the relay coil control signal and ECM supply during cranking and normal operation.
• Ensure there are no new codes indicating related power or ground faults.

Documentation

• Vehicle VIN, mileage, symptoms reported, and code data (P0686 with any related codes).
• All tests performed (voltages, coil resistance, control wire continuity, relay functional tests).
• Parts replaced (relay, fuses, wiring sections) and reasons.
• Road-test results and whether code reappeared after the repair.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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