P0691 Cooling Fan 1 Relay Control Circuit Low

The P0691 trouble code is a generic OBD-II diagnostic trouble code related to the electrical control circuit of the first engine cooling fan.

P0691 – Cooling Fan 1 Relay Control Circuit Low

This code means that the PCM/ECM has detected an unexpectedly low voltage or electrical condition in the Cooling Fan 1 relay control circuit.

The engine cooling fan is responsible for helping maintain the correct engine coolant temperature. Electric cooling fans are also commonly used to provide airflow through the A/C condenser, especially when the vehicle is stationary or traveling at low speed.

When the PCM commands Cooling Fan 1 on or off, it monitors the relay control circuit. If the electrical signal remains lower than expected, the PCM can store P0691.

Importantly, P0691 does not automatically mean that the cooling fan motor has failed. The actual problem may be the relay, wiring, connector, fuse/relay box, fan-control module, power supply, ground or PCM control circuit.

What Does P0691 Mean?

P0691 can be understood as:

P = Powertrain
0 = Generic OBD-II code
6 = Computer output/control-related fault
9 = Specific system identifier
1 = Cooling Fan 1 control circuit low

The key phrase is:

Cooling Fan 1 Relay Control Circuit Low

The PCM has detected an electrical condition in the Fan 1 relay control circuit that is lower than the expected value.

What Does "Circuit Low" Mean?

The word Low refers to the electrical signal or voltage detected by the PCM.

It does not necessarily mean that the fan is running at low speed.

Instead, the PCM is detecting a lower-than-expected voltage in the control circuit.

Possible causes include:

• Short to ground

• Faulty relay

• Damaged wiring

• Corroded connector

• Faulty fan-control module

• PCM driver problem

• Poor electrical connection

The exact interpretation depends on how the vehicle's cooling-fan circuit is designed.

What Is Cooling Fan 1?

Cooling Fan 1 is the fan or fan-control circuit designated as Fan 1 by the vehicle manufacturer.

Depending on the vehicle, it may be:

• The primary electric cooling fan

• One of two physical fans

• One speed of a multi-speed fan

• Part of an electronically controlled fan assembly

Cooling Fan 1 may operate according to:

• Engine coolant temperature

• A/C demand

• A/C refrigerant pressure

• Engine load

• Vehicle speed

• Ambient temperature

Because manufacturers use different configurations, the exact Fan 1 circuit should be identified using the vehicle-specific wiring diagram.

Symptoms of P0691

Possible symptoms include:

• Check Engine Light

• Cooling Fan 1 not operating

• Cooling fan operating intermittently

• Cooling fan running continuously

• Fan operating at the wrong speed

• Engine coolant temperature increasing

• Engine overheating

• A/C performance problems

• A/C compressor being disabled or limited

• Cooling fan cycling incorrectly

• Reduced engine performance in severe overheating conditions

Some vehicles may show few symptoms if another cooling fan can maintain acceptable engine temperature.

Can P0691 Cause Engine Overheating?

Yes.

If Cooling Fan 1 is required for adequate engine cooling and it does not operate, coolant temperature can rise.

The risk is especially high when:

• Idling

• Sitting in traffic

• Driving slowly

• Using the A/C

• Driving in hot weather

• Climbing hills

• Carrying heavy loads

If the coolant-temperature gauge approaches the overheating range, the vehicle should be stopped safely.

Common Causes of P0691

Faulty Cooling Fan 1 relay

The relay may have an internal electrical or mechanical fault.

Short to ground

The control wire may be unintentionally connected to ground, pulling the circuit voltage below the expected level.

Damaged wiring

A chafed, crushed or melted wire can create a short or poor connection.

Corroded connector

Corrosion can increase resistance or create unwanted electrical paths.

Loose relay terminal

A poor connection at the relay socket can interfere with normal circuit operation.

Damaged relay socket

Heat and corrosion can damage the terminals inside the fuse/relay box.

Incorrect relay

Installing the wrong relay can result in abnormal circuit behavior.

Blown fuse

A blown fuse can prevent the fan from operating, although it does not necessarily explain every circuit-low condition.

Faulty cooling fan motor

A failing or seized motor can affect the electrical system and may cause excessive current draw.

Faulty fan-control module

Some vehicles use an electronic module rather than a conventional relay.

Poor power or ground connection

A poor ground or power supply can interfere with proper fan operation.

PCM driver problem

The PCM may have an internal electronic driver responsible for controlling the relay.

Water intrusion

Moisture inside connectors, the fuse box or relay box can cause corrosion and short circuits.

Aftermarket electrical modifications

Additional accessories or previous wiring repairs can interfere with the fan circuit.

Is P0691 Serious?

It can be serious.

The code itself identifies an electrical fault, but the cooling fan is essential for maintaining engine temperature under many operating conditions.

If the fan does not operate correctly, continued driving can result in:

• Engine overheating

• Coolant boiling

• Head-gasket damage

• Cylinder-head damage

• Warped components

• Severe engine damage

The immediate risk depends on whether the remaining cooling system can keep the engine at a safe temperature.

Can You Drive With P0691?

If the engine remains at normal operating temperature and the cooling system is functioning correctly, a short drive to a repair facility may be possible.

However, do not continue driving if:

• The temperature gauge is rising

• The engine overheats

• The temperature warning light is on

• Cooling Fan 1 does not operate when required

• Coolant is boiling

• Steam is visible

• Coolant is leaking

If the engine overheats, continuing to drive can cause major engine damage.

P0691 vs. P0692

These codes concern the same Fan 1 control circuit but identify different electrical conditions.

P0691 – Cooling Fan 1 Relay Control Circuit Low

The PCM detects an unexpectedly low electrical condition.

P0692 – Cooling Fan 1 Relay Control Circuit High

The PCM detects an unexpectedly high electrical condition.

A circuit-low condition can point toward a short to ground or another problem pulling the control signal down.

P0691 vs. P0693

P0691 – Cooling Fan 1 Relay Control Circuit Low

Concerned with Fan 1.

P0693 – Cooling Fan 2 Relay Control Circuit Low

Concerned with Fan 2.

Both indicate a low electrical condition, but they refer to different cooling-fan circuits.

P0691 vs. P0694

P0691 – Cooling Fan 1 Relay Control Circuit Low

Fan 1, low circuit condition.

P0694 – Cooling Fan 2 Relay Control Circuit High

Fan 2, high circuit condition.

Can a Bad Cooling Fan Relay Cause P0691?

Yes.

A defective relay can create an abnormal control-circuit condition.

Possible relay failures include:

• Internal short

• Stuck contacts

• Open coil

• Incorrect resistance

• Heat damage

• Corrosion

The relay should be tested according to the manufacturer's specifications rather than replaced automatically.

Can a Short to Ground Cause P0691?

Yes.

A short to ground is one of the most important possible causes of a circuit-low condition.

If the Fan 1 relay control wire is unintentionally connected to ground, the PCM may detect a voltage lower than expected.

Inspect for:

• Chafed insulation

• Crushed wires

• Melted wiring

• Water intrusion

• Incorrect repairs

• Aftermarket modifications

Can a Bad Fan Motor Cause P0691?

Potentially.

The fan motor is not the direct definition of P0691, but a damaged motor can affect the electrical system.

A seized motor can draw excessive current and damage:

• Fuses

• Relays

• Wiring

• Fan-control modules

Can a Bad PCM Cause P0691?

Yes, but it should not be the first suspect.

The PCM may control the Fan 1 relay through an electronic output driver.

If this driver fails, the control circuit may not behave correctly.

Before replacing the PCM, check:

• Relay

• Wiring

• Connectors

• Fan motor

• Fan-control module

• PCM power

• PCM grounds

Can a Blown Fuse Cause P0691?

A blown fuse can stop the fan from operating, but a blown power fuse does not automatically explain a low relay-control code.

The fan's power circuit and relay control circuit should be tested separately.

Can P0691 Cause the Fan to Run Continuously?

Yes, depending on the vehicle's circuit design and the underlying fault.

A shorted circuit or defective relay can cause the fan to remain active.

Can P0691 Cause the Fan Not to Run?

Yes.

A low control signal can prevent the relay or fan-control module from operating the fan correctly.

Can P0691 Cause A/C Problems?

Yes, potentially.

Cooling fans often provide the airflow necessary to cool the A/C condenser.

If Fan 1 is unavailable, A/C performance may deteriorate, particularly at:

• Idle

• Low vehicle speeds

• Heavy traffic

• High ambient temperatures

Some vehicles may limit or disable the A/C system when cooling-fan operation is unavailable.

How to Diagnose P0691

A systematic electrical diagnosis is important.

1. Scan all modules

Look for additional codes involving:

• Cooling Fan 1

• Cooling Fan 2

• Engine coolant temperature

• A/C pressure

• Fan-control module

• Power supply

• Communication

Additional codes can help identify the root cause.

2. Check coolant temperature data

Use a scan tool to determine whether the coolant-temperature sensor is reporting a realistic value.

An incorrect temperature signal can affect fan operation.

3. Determine whether Fan 1 operates

Check whether the fan:

• Does not operate

• Runs continuously

• Operates intermittently

• Runs at the wrong speed

• Activates with the A/C

• Activates when the engine becomes hot

4. Command Fan 1 with a scan tool

If the vehicle supports active testing, command Fan 1 on.

Observe:

• Fan operation

• Relay operation

• Control-circuit voltage

• Fan current draw

5. Test the Fan 1 relay

Check:

• Correct relay type

• Coil resistance

• Contact operation

• Switching behavior

• Terminal condition

• Relay socket condition

6. Check relay power

Verify that the relay receives the specified battery voltage at the appropriate power terminals.

7. Measure the relay control circuit

Measure the control-circuit voltage while the PCM commands Fan 1 on and off.

The expected voltage depends on the circuit design.

Some vehicles switch the relay on the ground side, while others switch it on the positive side.

8. Check for a short to ground

Because P0691 indicates a low condition, inspect the control circuit for an unintended connection to ground.

Follow the manufacturer's specified test procedure.

9. Inspect the wiring

Look for:

• Chafed insulation

• Broken wires

• Melted wires

• Pinched wiring

• Corrosion

• Incorrect repairs

• Water damage

10. Inspect connectors

Check:

• Relay socket

• Fan connector

• Fuse box

• Fan-control module

• PCM connector

Look for:

• Loose terminals

• Bent pins

• Corrosion

• Burn marks

• Water intrusion

11. Test the cooling fan motor

Verify that the motor:

• Rotates freely

• Does not bind

• Does not make abnormal noises

• Draws the specified current

• Has a proper power supply and ground

12. Check the fan-control module

If an electronic module is used, verify:

• Power

• Ground

• Command signal

• Output

• Communication

13. Check PCM power and grounds

Poor PCM grounds can cause abnormal control-circuit behavior.

Voltage-drop testing is useful for identifying high-resistance ground connections.

14. Test the PCM control driver

If the relay, wiring, connectors, motor and fan-control module are all confirmed good, the PCM output driver may need to be tested.

PCM replacement should be considered only after the external circuit has been proven good.

How to Distinguish a Short to Ground From a Faulty Relay

If the relay is removed and the control wire still has an unwanted ground connection, the wiring may be responsible.

If the wiring tests correctly but the relay does not operate according to specifications, the relay becomes more suspicious.

If the relay operates correctly but the fan does not run, investigate:

• Fan power

• Fan ground

• Fan motor

• Fan-control module

How to Distinguish a Fan Motor Problem From a Control Problem

A fan motor problem becomes more likely when:

• Correct voltage reaches the motor

• Ground is good

• The motor does not operate

• Current draw is abnormal

• The motor is mechanically seized

A control problem becomes more likely when:

• The motor operates when properly powered

• The relay does not activate

• Relay control voltage is incorrect

• The PCM command does not reach the relay

Can P0691 Be Caused by Previous Wiring Repairs?

Yes.

Previous electrical work can introduce faults into the cooling fan circuit.

Examples include:

• Incorrectly grounded wires

• Poor crimp connections

• Incorrect relay wiring

• Added jumpers

• Damaged insulation

• Aftermarket accessory wiring

Can P0691 Return After Clearing the Code?

Yes.

If the underlying electrical problem remains, the PCM will monitor the circuit again and may set P0691 when the required conditions occur.

Clearing the code does not repair the circuit.

Common Diagnostic Mistakes With P0691

Replacing the cooling fan motor immediately

The motor may be functioning correctly.

Replacing the PCM immediately

The relay, wiring and connectors should be checked first.

Assuming "Low" means low fan speed

P0691 refers to the electrical control signal, not necessarily the physical fan speed.

Ignoring the wiring diagram

Cooling fan circuits differ significantly between vehicles.

Testing only continuity

Continuity alone may not identify a short to ground, high resistance or intermittent fault.

Ignoring the relay socket

Heat and corrosion can damage relay terminals.

Ignoring the fan-control module

Modern vehicles may use electronic fan control instead of a simple relay.

Ignoring coolant-temperature data

The PCM uses temperature information to determine when the cooling fan should operate.

What Is the Correct Repair for P0691?

The correct repair depends on the confirmed cause.

Possible repairs include:

• Replacing the Cooling Fan 1 relay

• Repairing a short to ground

• Repairing damaged wiring

• Repairing an open circuit

• Repairing corroded connectors

• Replacing damaged relay terminals

• Replacing a faulty cooling fan motor

• Replacing the fan-control module

• Repairing fan power or ground circuits

• Repairing PCM power or ground connections

• Updating PCM software where applicable

• Replacing the PCM if its internal fan-control driver is confirmed faulty

How to Test the Vehicle After Repair

After the repair, test the cooling system under several operating conditions.

Verify that:

• Fan 1 operates when commanded

• Fan 1 switches off when appropriate

• Fan speed is correct

• Engine coolant temperature remains normal

• A/C operates correctly

• Relay control voltage is within specification

• Fan current draw is normal

• No overheating occurs

• No additional cooling-fan codes appear

• P0691 does not return

Testing should ideally include idle operation, normal road driving and A/C operation.

Frequently Asked Questions About P0691

What does P0691 mean?

P0691 means the PCM has detected a low electrical condition in the Cooling Fan 1 relay control circuit.

Does P0691 mean the cooling fan is bad?

No. The relay, wiring, connectors, fan-control module or PCM may be responsible.

Can P0691 cause overheating?

Yes, if Fan 1 is required for adequate engine cooling and cannot operate correctly.

Can a bad relay cause P0691?

Yes.

Can a short to ground cause P0691?

Yes. It is one of the most important possible causes of a circuit-low condition.

Can a bad fan motor cause P0691?

Potentially, although the motor is not the direct definition of the code.

Can a bad PCM cause P0691?

Yes, but external components and wiring should be tested first.

Can P0691 cause the fan not to run?

Yes.

Can P0691 cause the fan to run continuously?

It can, depending on the underlying fault and the vehicle's circuit design.

Can P0691 cause A/C problems?

Yes. The cooling fan may be necessary for proper condenser cooling.

Can P0691 cause limp mode?

Some vehicles may activate a protective strategy if engine temperature becomes unsafe.

Can I drive with P0691?

Only if the engine maintains normal temperature and the cooling system is functioning correctly. If the temperature rises or the engine overheats, stop driving.

Does P0691 mean the cooling fan relay needs replacement?

Not necessarily. The relay and control circuit should be tested before replacement.

Final Thoughts on P0691

P0691 – Cooling Fan 1 Relay Control Circuit Low indicates that the PCM has detected an unexpectedly low electrical condition in the Cooling Fan 1 relay control circuit.

The most likely areas to investigate include:

• Cooling fan relay

• Control wiring

• Short to ground

• Relay socket

• Connectors

• Fan motor

• Fan-control module

• PCM power and ground

• PCM control driver

A logical diagnostic sequence is:

Scan All Modules → Check Coolant Temperature Data → Command Fan 1 On → Test Relay → Measure Control Circuit → Check for Short to Ground → Inspect Wiring → Test Fan Motor → Check Fan-Control Module → Verify PCM Power/Ground → Test PCM Driver

The important point is that P0691 does not automatically mean the cooling fan motor or PCM is defective.

Because a cooling fan fault can eventually cause engine overheating, the problem should be diagnosed promptly, particularly if Fan 1 is not operating or the engine temperature is increasing.