P0685 ECM/PCM Power Relay Control Circuit Malfunction

The P0685 – ECM/PCM Power Relay Control Circuit Malfunction diagnostic trouble code (DTC) indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a problem with the electrical circuit used to control the ECM/PCM power relay.

The power relay is responsible for supplying electrical power to the ECM/PCM and, depending on the vehicle, several other engine-management components. The control module activates and monitors this relay so that the engine-management system receives power when needed.

P0685 is a general power-relay control circuit code. Unlike P0687, which specifically indicates a high control-circuit condition, or P0689, which indicates low voltage in the relay sense circuit, P0685 can be set when the ECM/PCM detects a broader malfunction in the relay-control circuit.

What Does P0685 Mean?

In simple terms:

The ECM/PCM has detected that its power relay is not being controlled as expected.

A simplified system looks like:

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

The relay also has a control circuit:

ECM/PCM → Relay Control Circuit → Power Relay

When the ignition is switched on, the ECM/PCM commands the relay to operate.

If the expected electrical response is not detected, P0685 may be stored.

What Is the ECM/PCM Power Relay?

The ECM/PCM power relay is an electrically operated switch that supplies power to the vehicle's engine-management system.

Depending on the vehicle, it may provide power to:

• ECM/PCM

• Fuel injectors

• Ignition coils

• Engine sensors

• Emission-control solenoids

• Throttle-control components

• Transmission-related circuits

• Other engine actuators

The exact arrangement differs between manufacturers.

What Is the Power Relay Control Circuit?

The control circuit is the low-current circuit that operates the relay.

The ECM/PCM uses this circuit to activate or deactivate the relay.

A simplified example is:

ECM/PCM → Relay coil → Ground

When the ECM/PCM commands the relay on, the relay should switch and provide power to the required circuits.

If the control circuit is open, shorted, or otherwise malfunctioning, the ECM/PCM can set P0685.

What Causes P0685?

Possible causes include:

• Faulty ECM/PCM power relay

• Blown fuse

• Open relay-control circuit

• Shorted relay-control circuit

• Damaged wiring

• Corroded connector

• Loose relay terminal

• Damaged relay socket

• Faulty ignition switch

• Poor ECM/PCM ground

• Low battery voltage

• Incorrect relay

• Aftermarket electrical modification

• ECM/PCM internal driver failure

The actual cause should be confirmed through electrical testing.

Can a Bad Relay Cause P0685?

Yes.

A failed power relay is one of the possible causes.

The relay may have:

• Failed coil

• Burned contacts

• Internal corrosion

• Stuck contacts

• Incorrect resistance

• Mechanical failure

A relay can sometimes fail intermittently, making the vehicle difficult to diagnose.

Can a Blown Fuse Cause P0685?

Yes.

If the fuse supplies the ECM/PCM relay or its associated circuit, a blown fuse can prevent the relay from operating correctly.

However, do not simply replace the fuse and consider the problem solved.

A blown fuse may indicate:

• Short circuit

• Excessive current

• Damaged component

• Wiring fault

The reason the fuse failed should be identified.

Can a Broken Wire Cause P0685?

Yes.

An open wire in the relay-control circuit can prevent the ECM/PCM from operating the relay.

Inspect for:

• Broken wires

• Chafed insulation

• Melted wiring

• Corrosion

• Poor previous repairs

• Pinched harnesses

Can a Short to Ground Cause P0685?

Yes.

If the relay-control wire is shorted to ground, the ECM/PCM may be unable to control the relay correctly.

For example:

ECM control wire → Damaged insulation → Ground

This can prevent normal relay operation.

Can a Short to Battery Cause P0685?

Yes.

A control wire that receives battery voltage when it should not can also cause a relay-control malfunction.

This type of fault may produce a more specific high-voltage code, such as P0687, depending on the vehicle's monitoring strategy.

Can a Bad Relay Socket Cause P0685?

Yes.

The relay may be perfectly functional while its socket has:

• Loose terminals

• Corrosion

• Burned contacts

• Poor terminal tension

• Melted plastic

• Damaged internal wiring

A damaged relay socket can cause intermittent or permanent relay-control problems.

Can a Bad Ignition Switch Cause P0685?

Yes, on some vehicles.

The ignition switch can be part of the circuit responsible for powering or activating the ECM/PCM relay.

If the switch has an internal fault or high resistance, the relay may not operate as expected.

Can a Bad Ground Cause P0685?

Yes, potentially.

A poor ground can prevent the relay or ECM/PCM from operating correctly.

Check:

• Battery negative terminal

• Engine ground strap

• Chassis grounds

• ECM/PCM grounds

• Relay ground circuit

A voltage-drop test is preferable to relying only on a continuity test.

Can Low Battery Voltage Cause P0685?

Yes.

Low system voltage can interfere with relay operation and ECM/PCM control.

Possible causes include:

• Weak battery

• Poor battery connection

• Excessive starter current

• Faulty alternator

• Poor ground

Battery voltage should therefore be checked before replacing expensive components.

Can a Bad Alternator Cause P0685?

Potentially.

An alternator that is not charging properly can cause system voltage to fall.

This can result in unstable ECM/PCM operation.

The charging system should be tested if P0685 appears together with other voltage-related codes.

Can the ECM/PCM Cause P0685?

Yes, but it is not the first thing to suspect.

The ECM/PCM contains the electronic driver that controls the power relay on many vehicles.

If this driver fails, the relay may not receive the correct control signal.

However, external causes should be eliminated first.

Symptoms of P0685

Possible symptoms include:

• Check Engine Light

• Hard starting

• Extended cranking

• Intermittent no-start

• Engine stalling

• Complete no-start condition

• Reduced engine performance

• Transmission problems

• Multiple warning lights

• Intermittent electrical problems

The severity depends on whether the fault affects the actual ECM/PCM power supply or only the monitoring/control circuit.

Can P0685 Cause a No-Start Condition?

Yes.

This is one of the most important possible symptoms.

If the ECM/PCM power relay does not operate correctly, the ECM/PCM may not receive the power required to control the engine.

A possible sequence is:

Relay control fault → Power relay does not activate → ECM/PCM loses required power → Engine does not start

Can P0685 Cause Engine Stalling?

Yes.

If the relay or its control circuit intermittently fails while driving, power to the ECM/PCM can be interrupted.

This can cause:

• Sudden engine stall

• Loss of fuel injection

• Loss of ignition control

• Loss of throttle control

An intermittent P0685 problem should therefore not be ignored.

Can P0685 Cause Limp Mode?

Potentially.

The exact response depends on the vehicle.

If the ECM/PCM detects an important power-supply problem, it may activate a fail-safe operating strategy.

Can P0685 Cause Transmission Problems?

Yes.

On vehicles where the PCM controls the transmission, unstable PCM power can affect:

• Gear shifting

• Shift solenoids

• Torque-converter operation

• Transmission communication

Additional transmission DTCs may be stored.

How Is P0685 Diagnosed?

A systematic electrical diagnosis is recommended.

1. Scan all modules

Record:

• Stored DTCs

• Pending DTCs

• Communication codes

• Voltage-related codes

Do not diagnose P0685 in isolation if other codes are present.

2. Check the battery

Measure battery voltage and assess battery condition.

3. Check voltage during cranking

A significant voltage drop during starting can cause control-module problems.

4. Check charging voltage

Verify alternator operation.

5. Locate the ECM/PCM power relay

Use the vehicle-specific wiring diagram.

6. Inspect the relay

Look for:

• Burn marks

• Heat damage

• Corrosion

• Incorrect relay

• Physical damage

7. Test the relay

Measure coil resistance and verify contact operation.

8. Inspect the relay socket

Check terminal tension, corrosion, and heat damage.

9. Test the relay-control circuit

Measure the control voltage and verify that the ECM/PCM can command the relay.

10. Check for open circuits

Test the wiring between the relay and ECM/PCM.

11. Check for shorts

Test for unwanted connections to:

• Ground

• Battery voltage

• Other circuits

12. Check grounds

Verify ECM/PCM and relay grounds.

13. Test the ECM/PCM control output

If all external components and wiring are correct, evaluate the module's relay-control driver.

Can a Multimeter Diagnose P0685?

Yes.

A multimeter can be used to test:

• Battery voltage

• Cranking voltage

• Charging voltage

• Relay coil resistance

• Relay input voltage

• Relay output voltage

• Control-circuit voltage

• Voltage drop

• Continuity

• Shorts

The measurements should be compared with the vehicle manufacturer's specifications.

Why Is Voltage-Drop Testing Important?

A circuit can show continuity while still having excessive resistance.

For example:

Battery → Relay → ECM/PCM

A corroded terminal may pass a basic continuity test but create a significant voltage drop when the circuit is loaded.

Voltage-drop testing can identify:

• Poor relay contacts

• Corroded terminals

• Damaged wiring

• Weak grounds

• High-resistance connections

Can a Scan Tool Help?

Yes.

A scan tool can provide:

• System voltage

• ECM/PCM communication status

• Freeze-frame data

• Stored and pending DTCs

If the ECM intermittently disappears from the scan tool, investigate its power and ground circuits carefully.

What If P0685 Appears With P0687?

These codes are closely related.

P0685 is a general ECM/PCM power-relay control circuit malfunction.

P0687 specifically indicates a high condition in the control circuit.

If both appear, inspect:

• Relay

• Relay socket

• Control wire

• Short to battery

• ECM driver

What If P0685 Appears With P0688?

P0688 indicates an open condition in the ECM/PCM power-relay sense circuit.

If both codes are present, check the relay, relay socket, power circuit, sense circuit, and wiring together.

What If P0685 Appears With P0689 or P0690?

P0689 indicates a low sense-circuit condition, while P0690 indicates a high sense-circuit condition.

Multiple related codes can point toward a common relay, wiring, power-supply, or connector problem.

What If P0685 Appears With Many U-Codes?

Multiple U-codes can be a major clue.

A possible sequence is:

ECM/PCM power interruption → ECM communication loss → Other modules store U-codes

Do not immediately replace multiple control modules.

Check:

Battery → Relay → ECM/PCM power → Grounds → Communication network

What If P0685 Appears Only During Starting?

Focus on the starting electrical system.

Check:

• Battery condition

• Cranking voltage

• Battery terminals

• Engine ground

• Starter current draw

• ECM/PCM relay

• Relay control circuit

If the voltage drops excessively during cranking, the relay may not operate correctly.

What If P0685 Appears Intermittently?

An intermittent fault can be caused by:

• Loose relay

• Poor terminal contact

• Corroded connector

• Broken wire

• Ignition-switch problem

• Relay internal failure

• Poor ground

Inspect the harness and connectors while monitoring the relay circuit.

What If P0685 Appears After Battery Replacement?

Inspect:

• Battery terminals

• Positive cable

• Negative cable

• Engine ground

• Fuse box

• ECM/PCM relay

A connection that was disturbed during battery replacement can cause a new electrical fault.

What If P0685 Appears After Jump-Starting?

Check the complete electrical system.

Pay attention to:

• Battery

• Alternator

• ECM/PCM relay

• Fuses

• Grounds

• Wiring

If the code appeared immediately after a jump-start, electrical voltage should be checked carefully.

What If P0685 Appears After Engine Repair?

Inspect wiring and grounds around the engine.

Look for:

• Pinched harnesses

• Disconnected connectors

• Loose grounds

• Damaged relay wiring

• Incorrectly routed wires

Is P0685 a Serious Code?

It can be serious.

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

A persistent or intermittent fault can cause:

• No-start

• Engine stalling

• Loss of engine control

• Transmission problems

• Multiple electrical faults

Can You Drive With P0685?

If the vehicle starts and operates normally, a short trip to a repair facility may be possible.

However, driving should be avoided if you experience:

• Engine stalling

• Intermittent no-start

• Sudden loss of power

• Multiple warning lights

• ECM communication loss

A relay-control problem can potentially cause the engine to stop unexpectedly.

How Is P0685 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the ECM/PCM power relay

• Installing the correct relay

• Repairing damaged wiring

• Repairing an open control circuit

• Repairing a shorted control circuit

• Replacing damaged relay terminals

• Repairing the relay socket

• Cleaning corroded connectors

• Repairing ECM/PCM grounds

• Repairing the ignition-switch circuit

• Correcting aftermarket electrical modifications

• Repairing or replacing the ECM/PCM only when internal failure is confirmed

Does P0685 Require Relay Replacement?

Not necessarily.

The relay is only one possible cause.

The control circuit, relay socket, wiring, fuses, ignition switch, and ECM/PCM driver should all be tested.

Does P0685 Require ECM Replacement?

Usually not.

The ECM should be considered only after the external electrical system has been verified.

A sensible diagnostic order is:

Battery → Fuses → Relay → Relay socket → Control circuit → Wiring → Grounds → ECM/PCM

Common Diagnostic Mistakes

Replacing the ECM immediately

A relay, fuse, wire, or connector problem is much more common.

Replacing the relay without testing it

The relay may be functioning correctly.

Ignoring the relay socket

A loose or burned terminal can cause the same problem.

Checking only continuity

Voltage-drop testing can reveal high-resistance connections that continuity testing misses.

Ignoring battery and charging voltage

Low system voltage can affect relay operation.

Replacing a blown fuse without finding the cause

The underlying short or excessive-current condition may still be present.

Ignoring aftermarket electrical equipment

Poorly installed accessories can interfere with ECM/PCM power circuits.

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

These codes are related but describe different conditions.

P0685 – ECM/PCM Power Relay Control Circuit Malfunction

A general malfunction in the power-relay control circuit.

P0687 – ECM/PCM Power Relay Control Circuit High

The control circuit voltage is higher than expected.

P0688 – ECM/PCM Power Relay Sense Circuit/Open

The relay-sense circuit is open or interrupted.

P0689 – ECM/PCM Power Relay Sense Circuit Low

The sense-circuit voltage is lower than expected.

P0690 – ECM/PCM Power Relay Sense Circuit High

The sense-circuit voltage is higher than expected.

The important distinction is:

P0685 → General control-circuit malfunction

P0687 → Control circuit high

P0688 → Sense circuit open

P0689 → Sense circuit low

P0690 → Sense circuit high

How Do You Clear P0685?

The correct process is:

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

If P0685 returns, the underlying ECM/PCM power-relay control problem has not been properly corrected.

Final Thoughts on P0685

P0685 – ECM/PCM Power Relay Control Circuit Malfunction indicates that the ECM or PCM has detected a problem with the electrical circuit responsible for controlling its power relay.

Possible causes include a faulty relay, blown fuse, open or shorted wiring, damaged relay socket, loose connector, ignition-switch problem, poor ground, low system voltage, incorrect relay, aftermarket wiring, or, less commonly, an internal ECM/PCM driver failure.

The most important point is that P0685 is a general control-circuit code and does not automatically mean that the ECM/PCM itself is faulty.

The best diagnostic sequence is:

Check battery → Check fuses → Test relay → Inspect relay socket → Measure control circuit → Check for open/short → Inspect wiring → Check grounds → Verify ECM/PCM output.

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