P0687 – ECM/PCM Power Relay Control Circuit High

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

Definition source: SAE J2012/J2012DA (industry standard)

P0687 indicates the ECM/PCM power relay control circuit is being detected as “high” by the powertrain control system. In practical diagnostic terms, the control circuit signal is higher than expected for the current commanded state, which can occur with conditions such as a short-to-power, an open on the control side that allows the circuit to float high (varies by design), or a missing/weak ground path in the control circuit. Because relay strategies and feedback monitoring differ by vehicle, the same code may set under different operating conditions or with different accompanying symptoms. Always verify terminal identification, expected states, and test procedures using the applicable service information before making repairs.

What Does P0687 Mean?

P0687 – ECM/PCM Power Relay Control Circuit High means the control module has detected a high electrical condition on the circuit used to control the ECM/PCM power relay. The “circuit high” fault type refers to an electrical signal state (high input) rather than a confirmed failed relay or module. SAE J2012 defines the structure of DTCs and the general meaning of fault type wording, but the exact circuit routing (which side is switched, whether there is a feedback/monitor line, and how the relay is driven) varies by vehicle. Diagnostics should focus on proving why the control circuit is high when it should not be.

Quick Reference

• Subsystem: ECM/PCM power relay control circuit (power relay command/driver and any associated monitor/feedback path).
• Common triggers: Control circuit short-to-power, open/poor ground on the control side, incorrect relay wiring, or a control/feedback line stuck high.
• Likely root-cause buckets: Wiring/connector faults, power/ground distribution issues, relay coil/control circuit issues, module driver/logic issues (less common).
• Severity: Potentially high; may cause no-start, stalling, intermittent power loss to the control module, or multiple secondary DTCs.
• First checks: Battery condition, related fuses/fusible links, relay seating and terminal fit, visible harness damage near the relay/fuse box, and scan tool freeze-frame review.
• Common mistakes: Replacing the relay or control module without proving a circuit-high condition, ignoring ground integrity, and not checking for backfeed from aftermarket or shared circuits.

Theory of Operation

The ECM/PCM power relay supplies switched power to the control module and, on some designs, additional engine-management loads. The ECM/PCM typically commands the relay on or off using a dedicated control circuit through an internal driver, while the relay coil receives power or ground depending on the vehicle’s switching strategy. Some systems also monitor the commanded state using a feedback or sense circuit to confirm the relay control signal matches what is expected.

P0687 sets when the module detects the relay control circuit is at a high electrical state when it should not be (or remains high beyond what the monitor logic considers valid). This can be caused by a short-to-power, an open in the driver path that allows the circuit to float high (varies by vehicle), a poor ground that prevents the driver from pulling the circuit low, or a backfeed path keeping the circuit elevated.

Symptoms

• No-start: Engine may crank without starting if the control module’s power relay does not operate as intended.
• Stall: Engine may stall unexpectedly if module power is disrupted or relay control becomes erratic.
• Intermittent: Concern may come and go with vibration, temperature changes, or harness movement.
• Warning light: Malfunction indicator lamp may illuminate, sometimes with multiple related power/voltage codes.
• Reduced operation: Limited functionality or reduced performance may occur if the system enters a protective strategy.
• Hard restart: Extended crank or delayed restart may appear after a stall or key cycle.

Common Causes

• Short-to-power on the ECM/PCM power relay control circuit (harness chafe contacting a battery feed)
• Open ground on the relay control side (missing ground path causing the control line to remain pulled high)
• High resistance or poor terminal fit at the ECM/PCM connector affecting the relay control circuit (fretting, spread pins)
• High resistance or corrosion at the power relay connector causing abnormal control-circuit behavior
• Incorrect relay installed or relay with internal fault that backfeeds voltage onto the control circuit
• Aftermarket add-on wiring or remote-start/alarm splices backfeeding power into the relay control circuit
• Damaged fuse/relay box bussing or internal junction faults creating unintended voltage on the control line
• Faulty ECM/PCM driver circuit for the power relay control (only after wiring/relay integrity is verified)

Diagnosis Steps

Tools typically needed include a scan tool capable of reading freeze-frame data and live data, a digital multimeter, and (where service information supports it) a test light or relay tester. Use vehicle-specific wiring diagrams and connector views to identify the ECM/PCM power relay, its control circuit, and related power and ground paths. A back-probe kit and basic terminal inspection tools are also helpful.

1. Confirm the DTC and capture context: Verify P0687 is present. Record freeze-frame data and note any related power/voltage, ignition, or module-communication codes. Clear codes and see if P0687 returns immediately or only after a key cycle.
2. Perform a quick visual inspection: With the key off, inspect the fuse/relay box area and the harness routing to the ECM/PCM power relay. Look for rubbed-through insulation, pinched sections, non-factory splices, and signs of heat damage at the relay socket.
3. Check relay correctness and seating: Confirm the relay type/part is correct for the application (varies by vehicle) and fully seated. Swap with an identical known-good relay only if service information indicates it is safe to do so and the relay functions are equivalent.
4. Inspect connector condition at the relay and ECM/PCM: Disconnect the relay connector/socket access as applicable and the ECM/PCM connector(s) per service procedures. Look for corrosion, moisture intrusion, backed-out terminals, spread pins, or poor pin tension specifically on the relay control circuit and its related grounds.
5. Verify control circuit is not being backfed: With the relay removed (or control side isolated, varies by vehicle), use a multimeter to check whether the relay control circuit shows unwanted voltage presence when it should not. If voltage remains present with the relay removed, suspect a short-to-power or backfeed from another circuit/add-on wiring.
6. Check for short-to-power with continuity testing: Key off, disconnect the ECM/PCM connector that contains the relay control pin (per service info). Measure for continuity between the relay control wire and known battery feeds in the fuse/relay box area. Any unintended continuity indicates harness damage or an internal junction/box fault.
7. Check ground integrity and perform voltage-drop testing: Reconnect components as needed and command the relay on/off if your scan tool supports bi-directional control (varies by vehicle). Perform voltage-drop tests on the relay control ground path and ECM/PCM grounds during the commanded state. Excessive drop indicates resistance in ground connections, terminals, or splice packs.
8. Wiggle test while monitoring live data: Monitor live data PIDs related to ignition status, ECM/PCM power relay command (if available), and system voltage while gently moving the harness at the relay socket, fuse/relay box, and ECM/PCM connectors. If the fault sets or readings jump, isolate the exact movement point and inspect that section closely.
9. Validate relay coil/control behavior: If service information provides a test for the relay coil and control circuit, verify the relay coil is not internally shorted and that the control circuit can be pulled low/high as designed. Replace the relay only if it fails functional testing or shows internal backfeed behavior.
10. Isolate the circuit to pinpoint location: If P0687 persists, isolate the control circuit by disconnecting intermediate connectors/junctions (varies by vehicle). Re-test at each break to determine whether the high condition originates in the fuse/relay box side, the harness run, or at/near the ECM/PCM connector.
11. Consider ECM/PCM driver fault only after proving wiring: If the relay control circuit wiring, terminals, grounds, relay, and fuse/relay box paths test good and the circuit still indicates high when it should not, follow service information for ECM/PCM output driver testing. Replace or repair the module only after all external causes are eliminated.

Professional tip: If you suspect an intermittent short-to-power, log live data while reproducing the condition and focus on harness sections that move with engine torque or steering input. Many “circuit high” issues are caused by backfeeding through incorrect relays or non-factory splices, so confirm the control circuit is truly isolated when components are unplugged before condemning the ECM/PCM.

Possible Fixes & Repair Costs

Repair costs for P0687 vary widely by vehicle and depend on what testing confirms, parts access, and labor time. Avoid guessing; this is a “circuit high” electrical fault, so confirm the exact short/open/high-voltage condition before replacing components.

• Repair or replace the affected wiring section where a short-to-power, rubbed-through insulation, or incorrect splice is found
• Clean, tighten, and reseat connectors at the ECM/PCM power relay, fuse/relay box, and ECM/PCM; repair any damaged terminals or poor pin fit
• Replace the ECM/PCM power relay if it fails bench testing or shows intermittent sticking/high-resistance behavior tied to the fault
• Correct power feed distribution issues such as loose fuse box terminals, damaged fuse links, or compromised junction connections that backfeed voltage
• Perform voltage-drop testing and repair ground path issues that can make the control side read “high” due to an open/weak ground return
• Repair related circuit faults in the relay control driver line (including shorts to other powered circuits) verified by isolation testing
• As a last step after all external circuits test good, diagnose ECM/PCM driver/control issues per service information and replace/program only if confirmed

Can I Still Drive With P0687?

Driving with P0687 is not recommended if you have stalling, a no-start condition, sudden power loss, or warning indicators suggesting reduced engine control, because the ECM/PCM power relay circuit directly affects module power stability. If the vehicle runs normally and the code is stored without symptoms, limit driving to a short trip for diagnosis, avoid heavy traffic, and be prepared for an unexpected stall; verify charging system health and relay control integrity as soon as possible.

What Happens If You Ignore P0687?

Ignoring P0687 can lead to worsening intermittent faults, including sudden engine shutdowns, repeated no-start episodes, and loss of communication with control modules if the ECM/PCM power supply becomes unstable. Continued operation with an unresolved circuit-high condition can also complicate future diagnosis by creating multiple secondary codes and may increase the chance of electrical damage if a short-to-power or harness chafe progresses.

Related Relay Ecm/pcm Codes

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

• P0690 – ECM/PCM Power Relay Sense Circuit High
• P0891 – TCM Power Relay Sense Circuit High
• P0887 – TCM Power Relay Control Circuit High
• P0617 – Starter Relay Circuit High
• P0647 – A/C Clutch Relay Control Circuit High
• P0689 – ECM/PCM Power Relay Sense Circuit Low

Key Takeaways

• P0687 indicates the ECM/PCM power relay control circuit is being detected as “high,” consistent with a short-to-power or an open/weak ground path
• Start with wiring, connectors, and fuse/relay box inspection before considering relay or module replacement
• Confirm the condition with test-driven checks such as isolation testing, wiggle testing, and voltage-drop testing
• Symptoms can range from none to stalling/no-start because module power stability is involved
• Repair should target the verified cause; replacing parts without circuit confirmation often misses the root fault

Vehicles Commonly Affected by P0687

• Vehicles using an ECM/PCM power relay controlled by the ECM/PCM or an integrated power distribution module
• Vehicles with under-hood fuse/relay boxes exposed to heat, vibration, or moisture intrusion risk (design varies by vehicle)
• Vehicles with engine harness routing near sharp brackets, hot components, or moving parts that can cause insulation chafing
• Vehicles that have had recent electrical work (battery replacement, starter/alternator service, harness repairs) that may disturb relay control wiring
• Vehicles with aftermarket electrical accessories tied into ignition or relay feeds that can backfeed power (installation dependent)
• High-mileage vehicles where terminal tension loss, connector fretting, or internal relay wear is more likely
• Vehicles operated in corrosive environments where connector corrosion increases the chance of control-circuit faults
• Vehicles with multiple modules sharing power distribution paths where one circuit fault can influence relay control readings

FAQ

Does P0687 mean the ECM/PCM is bad?

No. P0687 only indicates the ECM/PCM power relay control circuit is being detected as “high.” Most confirmed causes are external to the module, such as short-to-power wiring damage, connector terminal issues, relay problems, or power/ground distribution faults; verify the circuit before suspecting the ECM/PCM.

What electrical problems typically create a “circuit high” condition for this code?

Common “circuit high” scenarios include a control wire shorted to a powered feed, backfeeding from another circuit, a damaged connector causing unintended contact, or an open/weak ground return that prevents the control side from being pulled low when commanded. Which applies depends on the specific relay control design.

Can a bad relay alone set P0687?

Yes, it can, if the relay’s coil/control behavior or internal faults cause the control circuit to remain at a high electrical level when the ECM/PCM expects a different state. However, a relay should be replaced only after confirming power, ground, and control wiring are correct and the relay fails functional testing.

Will clearing the code fix P0687?

Clearing the code does not fix the underlying electrical condition. If the circuit-high fault is still present, the monitor will typically fail again and the code will reset, sometimes immediately, other times after specific enable conditions are met (varies by vehicle).

What is the most efficient first step to diagnose P0687?

Start with a targeted visual and connector inspection at the ECM/PCM power relay, fuse/relay box, and harness routing, then perform circuit isolation to check for short-to-power/backfeed on the relay control line. Follow with a wiggle test and voltage-drop checks to find intermittent opens or poor grounds.

For an accurate repair plan, confirm whether the relay control is high due to a short-to-power, a backfeed, or a ground/control-path issue, then fix only what testing proves is faulty.