P0685 – ECM/PCM Power Relay Control Circuit/Open

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit/Open

Definition source: SAE J2012/J2012DA (industry standard)

DTC P0685 indicates the vehicle has detected an open circuit condition in the ECM/PCM power relay control circuit. In practical terms, this means the control path that commands the power relay for the engine controller is not electrically continuous when it should be, so the relay may not energize reliably or at all. Because relay control strategies, power distribution layouts, and monitoring logic vary by vehicle, the exact enabling conditions and what the controller considers a fault can differ. Use the correct wiring diagram, connector views, and test procedures from service information to verify the circuit path, terminal IDs, and expected relay command behavior before replacing parts.

What Does P0685 Mean?

P0685 – ECM/PCM Power Relay Control Circuit/Open means the powertrain controller has identified an open circuit in the control circuit used to operate the ECM/PCM power relay. The “open” fault type is an electrical integrity issue (loss of continuity) rather than a “high” or “low” signal condition. Per SAE J2012 DTC structure concepts, this code points you toward diagnosing the relay control circuit path (wiring, connectors, terminals, and related control elements) that the controller uses to switch the relay, and verifying that the relay command and circuit continuity are present when the controller expects them.

Quick Reference

• Subsystem: ECM/PCM power relay control circuit (relay command/control path feeding the engine controller power distribution).
• Common triggers: Open in relay control wire, disconnected/loose connector, poor terminal tension, damaged harness, or an inoperative relay control element (varies by vehicle).
• Likely root-cause buckets: Wiring/connector/terminal integrity; relay; power/ground distribution for the relay coil/control side; controller-side driver or command path (after circuit checks).
• Severity: Often high; may cause no-start, intermittent stall, or sudden loss of controller power depending on how the vehicle powers the ECM/PCM.
• First checks: Verify battery connections, fuses feeding the relay/control circuit, relay seating, connector engagement at relay/fuse box and controller, and visible harness damage.
• Common mistakes: Replacing the controller or relay without proving an open via continuity/voltage-drop testing and without inspecting terminal fitment and connector pin condition.

Theory of Operation

The ECM/PCM power relay is used to switch power to critical controller loads and related engine-management circuits. Typically, one side of the relay’s coil is supplied by a fused feed, and the other side is controlled by a command path that may be driven by the controller itself or by another control module (design varies by vehicle). When the relay is commanded on, the coil should energize, the relay contacts close, and the ECM/PCM receives stable operating power.

P0685 sets when the controller’s monitoring logic determines the relay control circuit is electrically open when a command is expected. An open can be caused by a broken conductor, backed-out terminal, poor pin fit, corrosion, an unplugged connector, or an internal open in the relay coil/control path. The result is that the relay may not energize, energizes intermittently, or drops out under vibration or load.

Symptoms

• No-start: Engine cranks but will not start, or no crank depending on how power is distributed.
• Stall: Engine may stall unexpectedly if controller power drops out.
• Intermittent operation: Vehicle may start and run normally, then fail after vibration, bumps, or heat soak.
• Warning indicators: Malfunction indicator lamp and/or other powertrain warnings may illuminate.
• Multiple codes: Additional unrelated-looking communication or sensor codes may appear due to controller power instability.
• Loss of throttle response: Reduced engine response may occur if the controller resets or enters a limited mode.

Common Causes

• Open circuit in the ECM/PCM power relay control wiring (broken conductor, pulled terminal, or damaged harness section)
• Poor connector fit or corrosion at the ECM/PCM power relay, relay socket, or ECM/PCM connector causing an open condition
• Relay control circuit pin backed out or spread terminal at the relay socket leading to intermittent or complete open
• Open in the relay coil feed or coil ground path (varies by vehicle design), preventing relay actuation
• Blown fuse or open fusible link supplying the relay control/coil circuit (power feed path open)
• Ignition switch or related feed circuit open upstream of the relay control/enable circuit (varies by vehicle)
• Open in the ECM/PCM power feed path through the relay contacts (relay output circuit open), causing the module to lose power and set the code
• ECM/PCM internal fault affecting the relay control driver or the ability to sense relay state (diagnose only after circuit integrity is proven)

Diagnosis Steps

Tools typically needed include a scan tool capable of reading codes and freeze-frame data, a digital multimeter, and basic back-probing leads. A test light can help confirm presence/absence of power, and a wiring diagram/service information is essential because relay control strategy and pin assignments vary by vehicle. If available, use a scan tool feature for data logging and output controls.

1. Confirm P0685 is present and record freeze-frame and any related DTCs. Pay attention to other power supply, ignition feed, or module communication codes, since loss of ECM/PCM power can create multiple secondary codes.
2. Verify the complaint. Check for no-start, intermittent stall, or power loss symptoms. If the vehicle will not communicate with the scan tool, treat that as a strong hint of an ECM/PCM power or ground issue and proceed with circuit checks.
3. Perform a visual inspection of the battery connections, primary grounds, and the harness routing to the power relay and ECM/PCM. Look specifically for damage that would create an open (rub-through, tugged wiring, recent repairs, unsecured connectors).
4. Locate the ECM/PCM power relay and its socket. With the key off, remove the relay and inspect relay blades and socket terminals for corrosion, heat damage, looseness, or signs of terminal push-out. Repair terminal fit issues before continuing.
5. Check all fuses and fusible links that supply the relay coil and relay contact (load) side using a power/continuity approach appropriate to the circuit. An open feed can mimic a control-circuit open depending on how the monitor is implemented; verify each feed is actually present at the relay socket.
6. Using service information, identify the relay coil control circuit and the coil feed/return path (varies by vehicle). With the key in the commanded-on condition, test for the expected power/ground presence at the relay coil terminals. If either side is missing, isolate whether the open is upstream (ignition/feed) or downstream (control/ground path).
7. Check continuity of the relay control wire(s) end-to-end between the relay socket and the ECM/PCM connector(s) per the wiring diagram. If continuity is lost, locate the open by inspecting splice points, in-line connectors, and harness bends; repair the open and retest.
8. Perform voltage-drop testing on the power and ground paths associated with the relay and ECM/PCM supply circuits while the circuit is loaded (engine cranking or relay commanded on). Excessive drop indicates high resistance that can behave like an open under load; correct the connection or cable issue found.
9. Wiggle test: with the relay commanded on (or during a controlled crank attempt), gently move the harness at the relay socket, fuse box, ignition feed area, and ECM/PCM connectors while monitoring scan tool data, a test light, or multimeter readings. If the state changes, focus on the last moved section for an intermittent open or poor pin fit.
10. If the relay and circuits test good, verify the relay itself by substitution with a known-good equivalent or by bench-testing the coil and contacts for proper operation. If the relay fails functional testing, replace it and recheck for proper power-up and code return.
11. As a final step only after wiring, terminals, feeds, grounds, and relay integrity are confirmed, evaluate ECM/PCM involvement. Confirm the module has stable power/ground and that the control circuit is intact; if the control driver is not commanding or sensing correctly per service information, further module diagnostics may be required.
12. Clear codes and perform a verification drive cycle or repeat the enabling conditions. Re-scan for P0685 and confirm the vehicle no longer exhibits power loss, no-start, or communication issues.

Professional tip: When diagnosing a circuit/open DTC, prioritize hands-on connector and terminal checks over assumptions. A relay socket terminal that looks normal can still be spread or partially backed out, creating an open only under vibration. Confirm repairs by recreating the original conditions (key cycles, crank, vibration) while logging data so intermittent opens don’t masquerade as fixed problems.

Possible Fixes & Repair Costs

Repair costs for P0685 vary widely because the root cause can range from a simple connection issue to a failed relay or module power feed problem. Total cost depends on diagnostic time, parts required, wiring accessibility, and labor complexity.

• Repair or replace open/loose terminals, damaged pins, or poor connector fit in the ECM/PCM power relay control circuit
• Repair broken, chafed, or corroded wiring in the relay control or relay power/ground paths (as applicable by vehicle design)
• Replace the ECM/PCM power relay if testing confirms an internal open or unreliable operation
• Restore proper power/ground supply to the relay and its control circuit (fuses, fusible links, junction connections, or ground points as applicable)
• Address ignition switch or related power distribution faults if they prevent relay control/supply from being present (varies by vehicle)
• Re-seat or repair ECM/PCM connectors if testing indicates an open at the module interface

Can I Still Drive With P0685?

P0685 can lead to a no-start, sudden stall, or intermittent loss of engine management power, so driving is not recommended if the engine stalls, cranks but will not start, or warning lamps indicate reduced control of critical functions. If the vehicle currently runs normally, limit operation to essential trips only and prioritize diagnosis, since an open in the ECM/PCM power relay control circuit can become an immediate shutdown condition without warning.

What Happens If You Ignore P0685?

Ignoring P0685 can result in progressively more frequent intermittent no-start or stalling events as an open circuit worsens or a marginal connection fully separates. Repeated power interruptions can also cause erratic module behavior, incomplete readiness monitors, and recurring fault memory, potentially leaving the vehicle unexpectedly inoperable.

Related Relay Ecm/pcm Codes

Compare nearby relay ecm/pcm trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P0688 – ECM/PCM Power Relay Sense Circuit/Open
• P0690 – ECM/PCM Power Relay Sense Circuit High
• P0689 – ECM/PCM Power Relay Sense Circuit Low
• P0687 – ECM/PCM Power Relay Control Circuit High
• P0686 – ECM/PCM Power Relay Control Circuit Low
• P0697 – Sensor Reference Voltage “C” Circuit/Open

Key Takeaways

• P0685 indicates an open condition in the ECM/PCM power relay control circuit, not a confirmed module failure by itself
• Opens are commonly caused by connector issues, terminal fit problems, wiring damage, or relay/control-side faults
• Diagnosis should focus on verifying relay command, circuit continuity, and voltage-drop under load rather than guessing parts
• Stalling or no-start risk makes this a high-priority electrical issue
• Fix only what testing proves, then verify by clearing codes and confirming stable operation

Vehicles Commonly Affected by P0685

• Vehicles that use an ECM/PCM power relay to supply key-on power to engine management circuits
• Vehicles with complex under-hood fuse/relay centers and multiple power distribution junctions
• Vehicles operated in environments that promote connector corrosion or moisture intrusion
• Vehicles with prior wiring repairs, aftermarket electrical additions, or non-original splices near power distribution
• High-mileage vehicles with heat-aged insulation and terminal tension loss in relay sockets
• Vehicles with frequent battery disconnects or low-voltage events that stress relay control operation
• Vehicles with engine compartments subject to vibration that can aggravate marginal terminal contact
• Vehicles with tight packaging where harnesses can rub through on brackets or covers

FAQ

Does P0685 mean the ECM/PCM is bad?

No. P0685 means the ECM/PCM power relay control circuit is detected as open. The cause is often wiring, connectors, relay socket terminal fit, power distribution, or relay/control-side faults. Confirm with circuit testing before suspecting a module.

Can a bad relay cause P0685?

Yes, if the relay has an internal open or an unreliable coil/connection that effectively creates an open in the control path. However, a relay that tests good on the bench may still fail in-vehicle due to socket tension or wiring opens, so test the entire circuit.

What is the difference between the relay control circuit and the relay power feed?

The control circuit is the path that energizes the relay (command and return path, depending on design), while the power feed side is the higher-current path the relay switches to supply the ECM/PCM and related loads. P0685 specifically points to an open in the control circuit, though related power/ground issues can contribute and should be verified during diagnosis.

Why does the vehicle sometimes start and other times not?

An open can be intermittent due to vibration, temperature changes, or connector/terminal movement. A marginal terminal fit, partially broken conductor, or corroded connection can make contact sometimes and go open at other times, causing inconsistent relay control and intermittent starting.

What should be checked first for P0685?

Start with visual and hands-on checks of the relay, relay socket/terminals, and related connectors and grounds, then confirm the relay control circuit integrity with continuity and voltage-drop testing under load. If the fault is intermittent, log data and perform a careful wiggle test to reproduce the open.

Always confirm the repair by clearing the code, verifying consistent starts, and rechecking for pending faults after a complete drive cycle under conditions that previously triggered the issue.