P0687 ECM/PCM Power Relay Control Circuit High

The P0687 – ECM/PCM Power Relay Control Circuit High diagnostic trouble code (DTC) indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected higher-than-expected voltage in the control circuit of the ECM/PCM power relay.

The ECM/PCM power relay is responsible for switching electrical power to the engine control system. The relay is controlled through a relatively low-current circuit, allowing the ECM/PCM to manage power to various engine and powertrain components.

When the voltage in the relay control circuit is higher than the expected value, the module can store P0687.

What Does P0687 Mean?

In simple terms:

The ECM/PCM detects excessive voltage in the circuit used to control its power relay.

A simplified system looks like:

Battery → Fuse → ECM/PCM Power Relay → ECM/PCM

The relay also has a control side:

ECM/PCM → Relay Control Circuit → Power Relay

The ECM/PCM monitors this control circuit.

If the voltage does not behave as expected and remains too high, P0687 can be triggered.

What Is the ECM/PCM Power Relay?

The ECM/PCM power relay is an electrically operated switch that controls power to the engine-management system.

Depending on the vehicle, it can supply power to:

• ECM/PCM

• Fuel injectors

• Ignition coils

• Engine sensors

• Emission-control solenoids

• Throttle-control components

• Other engine-management circuits

The exact arrangement varies by manufacturer.

What Is the Power Relay Control Circuit?

The control circuit is the low-current side of the relay.

The ECM/PCM uses this circuit to switch the relay on or off.

A simplified example is:

ECM/PCM control output → Relay coil → Ground

When the ECM/PCM wants the relay activated, it controls the relay coil.

If the electrical conditions in this circuit are abnormal, the ECM/PCM can detect the problem.

With P0687, the control circuit is detected as too high.

What Causes P0687?

Possible causes include:

• Short to battery voltage

• Faulty ECM/PCM power relay

• Incorrect relay

• Damaged control wire

• Incorrect wiring repair

• Corroded connector

• Damaged relay socket

• Faulty ignition-switch circuit

• Internal relay-coil problem

• Aftermarket electrical modification

• ECM/PCM driver circuit failure

The exact cause depends on the vehicle's electrical design.

Can a Short to Battery Cause P0687?

Yes.

A short to battery voltage is one of the most important possibilities.

If the relay control wire is supposed to be controlled by the ECM/PCM but becomes connected to battery voltage, the module can detect an abnormally high voltage.

For example:

Battery voltage → Damaged control wire → Relay control circuit → P0687

Inspect the harness carefully for:

• Chafing

• Melted insulation

• Pinched wires

• Incorrect repairs

• Contact with power wires

Can a Bad Relay Cause P0687?

Yes.

A defective relay can create abnormal conditions in the control circuit.

Possible problems include:

• Incorrect coil resistance

• Internal short

• Damaged coil

• Incorrect relay

• Internal corrosion

• Stuck contacts

The relay should be tested according to the manufacturer's specifications.

Can an Incorrect Relay Cause P0687?

Yes.

A relay can physically fit into the socket while having an incorrect internal configuration.

This can change the expected electrical behavior of the control circuit.

Always verify:

• Part number

• Pin configuration

• Coil specifications

• Relay type

before replacing the relay.

Can a Bad Relay Socket Cause P0687?

Yes.

A damaged relay socket can cause abnormal electrical behavior.

Inspect for:

• Loose terminals

• Burned terminals

• Corrosion

• Melted plastic

• Poor terminal tension

• Damaged internal wiring

Can a Bad Ignition Switch Cause P0687?

Potentially.

On some vehicles, the ignition switch is involved in the ECM/PCM power-relay circuit.

A faulty switch or wiring problem can apply voltage to the circuit when it should not be present.

Can a Bad Alternator Cause P0687?

Potentially, but it is not the first suspect.

If the charging system produces excessive system voltage, multiple electrical circuits can experience abnormal voltage.

Check the charging system if P0687 occurs with other high-voltage or electrical DTCs.

Can a Bad Battery Cause P0687?

A weak battery is generally associated with low-voltage conditions rather than P0687.

However, the entire electrical system should still be checked because unstable system voltage can produce unusual symptoms.

Can a Bad Ground Cause P0687?

Possibly.

A poor ground can alter voltage references and cause the control circuit to behave unexpectedly.

Check:

• Battery grounds

• Engine ground strap

• Chassis grounds

• ECM/PCM grounds

Can the ECM/PCM Cause P0687?

Yes, but it is less common.

The ECM/PCM controls the relay through an internal electronic driver.

If this driver becomes defective, the relay control circuit may remain at an incorrect voltage.

However, an ECM/PCM should not be replaced before checking the external circuit.

Symptoms of P0687

Possible symptoms include:

• Check Engine Light

• Hard starting

• Extended cranking

• Intermittent no-start

• Engine stalling

• Engine may not start

• Reduced engine performance

• Transmission problems

• Multiple warning lights

• Intermittent electrical problems

In some vehicles, the only obvious symptom may be the Check Engine Light.

Can P0687 Cause a No-Start Condition?

Yes.

If the power relay cannot be controlled correctly, the ECM/PCM or other engine-management components may not receive the required power.

A possible sequence is:

Relay control fault → Power relay does not operate correctly → ECM/PCM power problem → Engine cannot start

Can P0687 Cause Engine Stalling?

Yes, potentially.

If the relay control circuit intermittently malfunctions while driving, the power relay can switch incorrectly.

This may interrupt power to the ECM/PCM and cause:

• Engine stall

• Loss of fuel injection

• Loss of ignition control

• Loss of throttle control

Can P0687 Cause Limp Mode?

Potentially.

If the control module detects a serious power-relay problem, it may activate a fail-safe operating strategy.

The exact response depends on the vehicle.

Can P0687 Cause Transmission Problems?

Yes.

If the PCM controls the transmission, an unstable power supply can affect:

• Gear shifting

• Shift solenoids

• Torque converter operation

• Transmission communication

Additional transmission DTCs may be stored.

How Is P0687 Diagnosed?

A systematic electrical diagnosis is recommended.

1. Scan all control modules

Record:

• Stored DTCs

• Pending DTCs

• Communication codes

• Voltage-related codes

Other codes may help identify the source.

2. Check battery voltage

Verify that the battery is in good condition.

3. Check charging voltage

Make sure the alternator is not producing excessive voltage.

4. Identify the ECM/PCM power relay

Use the vehicle-specific wiring diagram.

5. Inspect the relay

Check for:

• Burn marks

• Heat damage

• Corrosion

• Incorrect relay

• Physical damage

6. Test the relay

Measure coil resistance and verify switching operation.

7. Measure the control-circuit voltage

Compare the actual voltage with the manufacturer's specifications.

8. Check for a short to battery

Determine whether battery voltage is entering the control circuit when it should not.

9. Inspect the relay socket

Look for loose, damaged, or corroded terminals.

10. Inspect the wiring harness

Pay particular attention to areas exposed to:

• Heat

• Vibration

• Moving components

• Previous repairs

11. Check ECM/PCM grounds

Perform voltage-drop testing where appropriate.

12. Test the ECM/PCM control output

If all external components are correct, evaluate the ECM/PCM driver circuit.

Can a Multimeter Diagnose P0687?

Yes.

A multimeter can be used to test:

• Battery voltage

• Charging voltage

• Relay coil resistance

• Relay control voltage

• Voltage at the relay

• Voltage drop

• Continuity

• Short to battery voltage

The readings must be compared with the manufacturer's specifications.

Why Is Voltage-Drop Testing Important?

A circuit may appear electrically continuous but still have a problem under load.

Voltage-drop testing can identify:

• High resistance

• Poor terminals

• Corroded connectors

• Damaged wiring

• Weak grounds

For electrical relay problems, this can be more informative than a simple continuity test.

Can a Scan Tool Help?

Yes.

A scan tool can provide:

• System voltage

• ECM/PCM communication status

• Freeze-frame information

• Stored and pending DTCs

Some vehicles may also provide data showing relay or power-management status.

What If P0687 Appears With P0689 or P0690?

This combination should be taken seriously.

P0687 concerns the control circuit, while P0689 and P0690 concern the sense circuit.

If several of these codes appear together, investigate:

• Power relay

• Relay socket

• Battery voltage

• Wiring

• Grounds

• ECM/PCM power supply

Do not assume that multiple components have failed independently.

What If P0687 Appears With Many U-Codes?

Multiple communication codes can indicate a common power problem.

For example:

Power relay fault → ECM/PCM power interruption → Network communication loss → Multiple U-codes

Check the main power and ground circuits before replacing control modules.

What If P0687 Appears Intermittently?

An intermittent control-circuit problem may be caused by:

• Loose relay

• Loose terminal

• Corroded connector

• Damaged wire

• Harness vibration

• Ignition-switch problem

• Relay failure

Wiggle testing the harness while monitoring the circuit can sometimes help locate the fault.

What If P0687 Appears After Battery Replacement?

Check:

• Battery terminals

• Positive cable

• Negative cable

• Ground connections

• Fuse box

• ECM/PCM relay

• Relay socket

A loose connection can create electrical problems after battery service.

What If P0687 Appears After Jump-Starting?

Inspect:

• Battery

• Charging system

• Power relay

• Fuses

• Grounds

• ECM/PCM wiring

If the code appeared immediately after a jump-start, check the electrical system carefully.

What If P0687 Appears After Engine Repair?

Inspect the engine-bay harness and grounds.

Pay special attention to:

• ECM connectors

• Ground straps

• Relay wiring

• Battery cables

• Harness routing

A wire can be pinched or damaged during mechanical work.

Is P0687 a Serious Code?

It can be serious.

The ECM/PCM power relay is essential to engine operation.

A control-circuit fault can potentially cause:

• No-start

• Engine stalling

• Loss of engine control

• Transmission problems

• Intermittent electrical failures

If the vehicle is stalling or losing power, diagnosis should not be delayed.

Can You Drive With P0687?

If the vehicle operates normally and P0687 is only stored, a short trip to a repair facility may be possible.

However, avoid driving if there is:

• Engine stalling

• Sudden loss of power

• Intermittent no-start

• Multiple warning lights

• ECM communication loss

An intermittent relay-control problem can potentially cause the engine to shut down unexpectedly.

How Is P0687 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the ECM/PCM power relay

• Installing the correct relay

• Repairing a shorted control wire

• Repairing damaged wiring

• Replacing damaged terminals

• Repairing the relay socket

• Cleaning corroded connectors

• Repairing an ignition-switch circuit

• Correcting aftermarket wiring

• Repairing an ECM/PCM ground

• Repairing the ECM/PCM driver circuit

• Replacing the ECM/PCM only when internal failure is confirmed

Does P0687 Require Relay Replacement?

Not necessarily.

The relay is only one possible cause.

A short to battery voltage in the control wire, a damaged relay socket, or an ECM driver problem can also produce the code.

Does P0687 Require ECM Replacement?

Usually not.

The ECM should be considered only after the external circuit has been thoroughly tested.

A sensible diagnostic order is:

Battery → Charging system → Fuse → Relay → Relay socket → Control wire → Grounds → ECM/PCM output

Common Diagnostic Mistakes

Replacing the ECM immediately

The actual problem may be a shorted wire or faulty relay.

Replacing the relay without testing the control circuit

A short to battery voltage can remain after the relay is replaced.

Ignoring the relay socket

Damaged terminals can create intermittent electrical problems.

Ignoring the charging system

Excessive system voltage can affect multiple circuits.

Checking only continuity

Voltage-drop and loaded-circuit testing are often more useful.

Ignoring aftermarket accessories

Poorly installed electrical equipment can interfere with the relay-control circuit.

P0685 vs. P0687 vs. P0688 vs. P0689 vs. P0690

These codes are closely related but describe different parts or conditions of the ECM/PCM power-relay circuit.

P0685 – ECM/PCM Power Relay Control Circuit/Open

Indicates a general open or malfunction in the power-relay control circuit.

P0687 – ECM/PCM Power Relay Control Circuit High

Indicates that the relay control circuit voltage is higher than expected.

P0688 – ECM/PCM Power Relay Sense Circuit/Open

Indicates an open or interrupted relay-sense circuit.

P0689 – ECM/PCM Power Relay Sense Circuit Low

Indicates lower-than-expected voltage in the relay-sense circuit.

P0690 – ECM/PCM Power Relay Sense Circuit High

Indicates higher-than-expected voltage in the relay-sense circuit.

The key distinction is:

P0685 → Control circuit malfunction/open

P0687 → Control circuit voltage too high

P0688 → Sense circuit open

P0689 → Sense circuit voltage too low

P0690 → Sense circuit voltage too high

How Do You Clear P0687?

The proper process is:

Scan all modules → Record DTCs → Check battery → Check charging system → Identify ECM/PCM relay → Test relay → Measure control-circuit voltage → Check for short to battery → Inspect relay socket → Inspect wiring → Check grounds → Verify ECM/PCM control output → Repair the confirmed fault → Clear DTCs → Start vehicle → Road test → Rescan

If P0687 returns, the high-voltage condition in the ECM/PCM power-relay control circuit has not been properly corrected.

Final Thoughts on P0687

P0687 – ECM/PCM Power Relay Control Circuit High indicates that the ECM or PCM has detected higher-than-expected voltage in the circuit used to control its power relay.

Possible causes include a short to battery voltage, faulty or incorrect relay, damaged wiring, relay-socket problems, ignition-switch faults, poor electrical connections, aftermarket wiring, or, less commonly, an internal ECM/PCM driver failure.

The most important point is that P0687 does not automatically mean the ECM/PCM is defective.

The best diagnostic sequence is:

Check battery → Check charging system → Inspect relay → Test relay socket → Measure control voltage → Check for short to battery → Inspect wiring → Check grounds → Test ECM/PCM control output.

If the vehicle has intermittent stalling, no-start problems, or sudden loss of engine power, the fault should be diagnosed promptly because an ECM/PCM power-relay problem can potentially interrupt engine operation.