P0945 Hydraulic Pump Relay/Control Circuit/Open

The P0945 trouble code is an OBD-II diagnostic trouble code related to the electrical circuit that controls the transmission's hydraulic pump.

P0945 – Hydraulic Pump Relay/Control Circuit/Open

This code means that the Transmission Control Module (TCM) or Powertrain Control Module (PCM) has detected an open circuit or an interruption in the electrical circuit used to control the hydraulic pump relay.

In simple terms, the transmission computer is attempting to control or monitor the hydraulic pump, but it detects that the electrical circuit is not complete.

The problem may involve:

• Hydraulic pump relay

• Pump control wiring

• Power supply

• Ground

• Fuse

• Electrical connector

• Hydraulic pump

• Pump control module

• TCM

Importantly, P0945 does not automatically mean that the hydraulic pump itself has failed.

What Does P0945 Mean?

P0945 – Hydraulic Pump Relay/Control Circuit/Open

Automatic transmissions depend on hydraulic pressure to operate many of their internal functions.

Hydraulic pressure can control:

• Clutch packs

• Shift valves

• Brakes

• Gear engagement

• Pressure-control circuits

• Other hydraulic actuators

Depending on the transmission design, an electric hydraulic pump may be controlled through a relay or an electronic control circuit.

A simplified system can be represented as:

TCM → Relay/control circuit → Hydraulic pump → Hydraulic pressure → Transmission operation

If the electrical circuit between these components is interrupted, the TCM may detect an open circuit and store P0945.

What Does “Open Circuit” Mean?

An open circuit means that electrical current cannot travel through the intended circuit.

Think of it as a broken electrical path.

An open circuit can be caused by:

• Broken wire

• Disconnected connector

• Failed relay

• Blown fuse

• Damaged terminal

• Corroded connection

• Poor electrical contact

• Broken ground

• Internal pump electrical failure

• Damaged control module

The exact location of the open circuit must be determined through electrical testing.

What Is the Hydraulic Pump Relay?

A relay is an electrically controlled switch.

It allows the transmission control system to activate a component that may require more current than the TCM can safely provide directly.

A simplified circuit can look like:

TCM control → Relay → Pump power → Hydraulic pump

When the TCM requests pump operation, the relay switches power to the pump.

If the relay circuit is open, the pump may not receive the required electrical power.

What Is the Hydraulic Pump?

The hydraulic pump creates the fluid pressure required for transmission operation.

In traditional automatic transmissions, the main pump is often mechanically driven.

However, some modern transmissions use electric hydraulic pumps, auxiliary pumps or electro-hydraulic systems.

These can be used to:

• Maintain hydraulic pressure

• Support transmission operation when the engine is stopped

• Assist stop-start systems

• Support hybrid operation

• Operate electro-hydraulic clutch systems

• Maintain pressure during specific operating conditions

Because transmission designs vary, the exact function of the pump controlled by the circuit can differ between vehicles.

Does P0945 Mean the Pump Is Bad?

No.

P0945 specifically identifies an open relay/control circuit.

The pump itself is only one possible cause.

The actual problem may be much simpler, such as:

• Disconnected connector

• Broken wire

• Blown fuse

• Faulty relay

• Corroded terminal

Replacing the pump without testing the circuit can result in an unnecessary repair.

Symptoms of P0945

The symptoms depend on the vehicle and transmission design.

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Delayed gear engagement

• Delayed Drive engagement

• Delayed Reverse engagement

• Harsh shifting

• Transmission slipping

• Limited gear operation

• Limp mode

• Hydraulic pump not operating

• Pump-related noise

• Transmission overheating

• Loss of Drive

• Loss of Reverse

• Vehicle refusing to move

Some vehicles may continue operating normally for a short period if another hydraulic pressure source is available.

Common Causes of P0945

Failed hydraulic pump relay

The relay may have an internal open circuit.

Possible problems include:

• Burned contacts

• Broken relay coil

• Internal corrosion

• Mechanical failure

• Internal open circuit

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

Blown fuse

A blown fuse can interrupt power to the relay or pump.

However, the cause of the blown fuse should also be determined.

If the fuse repeatedly fails, there may be:

• Short circuit

• Pump motor failure

• Wiring damage

• Excessive current draw

Broken wiring

A broken wire is one of the most direct causes of an open circuit.

Wiring can become damaged through:

• Chafing

• Heat exposure

• Vibration

• Pinching

• Mechanical impact

• Incorrect previous repairs

Disconnected connector

A connector that is disconnected or not fully seated can interrupt the circuit.

This is particularly important after:

• Transmission repair

• Transmission replacement

• Electrical work

• Battery service

• Underbody repairs

Corroded terminals

Corrosion can prevent electrical current from flowing through a connector.

Look for:

• Green or white corrosion

• Rust

• Moisture

• Burn marks

• Loose terminals

Loose terminal

A terminal may be pushed backward inside the connector.

The connector may appear fully installed while the electrical contact remains incomplete.

Damaged ground

The pump relay or pump may require a reliable ground connection.

A broken ground can create an open-circuit condition.

Faulty electric hydraulic pump

If the pump motor has an internal open circuit, the control system may detect that the expected electrical response is missing.

Possible internal problems include:

• Broken motor winding

• Internal connector failure

• Internal thermal protection failure

• Pump controller failure

Faulty pump control module

Some systems use a dedicated electronic controller for the pump.

A failure in that module can interrupt communication or pump operation.

TCM fault

A TCM problem is possible but should generally be considered only after the external circuit has been checked.

Can Low Transmission Fluid Cause P0945?

Low transmission fluid does not normally cause an electrical open-circuit code.

Low fluid can cause:

• Low hydraulic pressure

• Pump cavitation

• Slipping

• Delayed engagement

• Transmission overheating

However, these are hydraulic problems rather than an electrical circuit interruption.

The fluid level should still be checked because hydraulic problems may exist alongside P0945.

Can Dirty Transmission Fluid Cause P0945?

Dirty transmission fluid does not normally cause P0945 directly.

Contaminated fluid can affect:

• Pump operation

• Valve-body operation

• Hydraulic pressure

• Solenoids

• Clutches

But it does not normally create an electrical open circuit.

If P0945 is stored, the electrical circuit should be investigated first.

P0945 vs. P0946

These codes identify different types of circuit problems.

P0945 – Hydraulic Pump Relay/Control Circuit/Open

The circuit is open or electrically interrupted.

P0946 – Hydraulic Pump Relay Circuit Range/Performance

The circuit operates, but its performance is outside expected parameters.

P0945 is therefore more directly associated with a loss of electrical continuity.

P0945 vs. P0947

P0945 – Hydraulic Pump Relay/Control Circuit/Open

The electrical circuit is interrupted.

P0947 – Hydraulic Pump Relay Circuit Low

The circuit voltage or signal is lower than expected.

A broken wire may produce an open circuit, while a short to ground or high-resistance connection may produce a low-voltage condition.

P0945 vs. P0948

P0945 – Hydraulic Pump Relay/Control Circuit/Open

The circuit is interrupted.

P0948 – Hydraulic Pump Relay/Control Circuit High

The circuit voltage or signal is higher than expected.

A short to battery voltage or an incorrect power feed can contribute to P0948.

How Serious Is P0945?

P0945 can be a serious transmission fault.

The seriousness depends on the role of the hydraulic pump in the specific transmission.

If the pump cannot operate because its control circuit is open, the transmission may not be able to generate or maintain the hydraulic pressure required for proper operation.

This can lead to:

• Loss of gear engagement

• Slipping

• Harsh shifting

• Limp mode

• Clutch damage

• Loss of Drive or Reverse

Can You Drive With P0945?

Driving with P0945 is not recommended if the transmission is showing symptoms.

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

However, stop driving if you experience:

• Loss of Drive

• Loss of Reverse

• Severe slipping

• Violent shifting

• Significant engagement delay

• Transmission overheating

• Limp mode

Continuing to drive with insufficient hydraulic pressure can potentially cause additional transmission damage.

How to Diagnose P0945

A proper diagnosis should determine where the electrical circuit is open.

1. Scan all control modules

Use a professional diagnostic scanner to check:

• TCM

• PCM

• Body-control module

• Transmission-related modules

Record all stored and pending codes.

Additional codes may reveal the underlying problem.

2. Check freeze-frame data

Review the operating conditions when P0945 was stored.

Look for:

• Engine RPM

• Vehicle speed

• Transmission temperature

• Selected gear

• System voltage

• Pump command

• Pump status

3. Inspect the hydraulic pump relay

Check:

• Relay condition

• Power supply

• Ground

• Control signal

• Output

• Coil resistance

If the relay is faulty, replace it with the correct specification.

4. Check the relevant fuse

Inspect the pump-related fuse.

If the fuse is blown, determine why it failed before replacing it.

5. Check relay power

Verify that the relay is receiving the required power supply.

An absent power supply can make a good relay appear defective.

6. Check relay control

Verify that the TCM or appropriate control module is providing the expected control signal.

7. Inspect wiring

Trace the complete circuit between:

• TCM

• Relay

• Pump

• Ground

• Power supply

Look for:

• Broken wires

• Chafed insulation

• Melted sections

• Pinched harnesses

• Incorrect repairs

8. Test continuity

With the appropriate circuit safely isolated, check continuity through the relevant wiring.

An open circuit should show a clear interruption in the expected path.

9. Perform voltage-drop testing

If the circuit appears intact, voltage-drop testing can identify hidden resistance.

Check:

• Power side

• Ground side

• Relay contacts

• Pump power supply

10. Inspect connectors

Check for:

• Corrosion

• Bent pins

• Loose terminals

• Backed-out terminals

• Water intrusion

• Damaged connector locks

11. Test the electric hydraulic pump

If the vehicle uses an electric pump, check:

• Power supply

• Ground

• Motor resistance

• Current draw

• Activation

• Pump operation

Follow the manufacturer's specifications.

12. Check hydraulic pressure

If the transmission design allows pressure testing, measure hydraulic pressure according to the manufacturer's procedure.

This determines whether the electrical fault is also creating a real hydraulic-pressure problem.

13. Inspect transmission fluid

Check:

• Fluid level

• Correct specification

• Fluid condition

• Leaks

• Contamination

Fluid should be checked using the correct manufacturer procedure and temperature range.

14. Evaluate the TCM

If the wiring, relay, fuse, pump and power supply are all confirmed to be correct, the TCM can be investigated.

The TCM should not be replaced simply because P0945 is present.

Why P0945 Can Be Difficult to Diagnose

An open circuit can sometimes be straightforward to find.

However, intermittent open circuits can be much more difficult.

For example, a damaged wire may:

• Work when stationary

• Open when the harness moves

• Work when cold

• Fail when hot

• Make contact over bumps

• Lose contact due to vibration

In these situations, technicians may need to perform a wiggle test, voltage monitoring or other dynamic circuit tests.

P0945 After Transmission Repair

If P0945 appeared immediately after transmission work, inspect all wiring and connectors that were disturbed.

Pay particular attention to:

• Pump connectors

• Relay connectors

• Transmission harness

• Ground straps

• Power connections

A partially connected connector is a common and easily overlooked possibility.

P0945 After Transmission Replacement

After transmission replacement, verify:

• Correct wiring

• Correct connectors

• Proper ground connections

• Correct relay

• Correct fuse

• Correct pump configuration

The harness should also be checked for damage or pinching during installation.

P0945 After Battery Replacement

Battery replacement does not normally cause P0945 directly.

However, if the code appeared afterward, inspect:

• Battery terminals

• Main grounds

• Power connections

• Fuses

• Charging-system operation

A loose connection can cause multiple electrical faults.

Can P0945 Cause Limp Mode?

Yes.

If the transmission control system cannot operate the hydraulic pump correctly, it may activate a protective operating strategy.

Possible results include:

• Limp mode

• Limited gear selection

• Harsh shifting

• Warning lights

• Reduced transmission performance

The exact behavior depends on the vehicle.

Can P0945 Damage the Transmission?

The electrical fault itself does not mechanically damage the transmission.

The problem is what can happen if the hydraulic pump cannot operate.

Insufficient hydraulic pressure can lead to:

• Clutch slipping

• Excessive heat

• Poor clutch engagement

• Abnormal shifting

• Increased internal wear

Continued driving under these conditions can potentially turn an electrical problem into a major transmission repair.

Common Diagnostic Mistakes With P0945

One common mistake is immediately replacing the hydraulic pump.

P0945 indicates an open relay/control circuit, not necessarily a failed pump.

Another mistake is replacing the relay without checking the fuse and wiring.

If the relay is not receiving power, replacing the relay will not solve the problem.

Another mistake is ignoring ground connections.

A broken ground can create the same symptoms as an open power circuit.

Another mistake is performing only a visual inspection.

A wire can look intact externally while being broken internally.

What Is the Correct Repair for P0945?

The correct repair depends on where the open circuit is found.

Possible repairs include:

• Replacing the hydraulic pump relay

• Replacing a blown fuse after correcting the underlying fault

• Repairing broken wiring

• Replacing damaged terminals

• Repairing corroded connectors

• Repairing ground connections

• Restoring power supply

• Repairing pump wiring

• Replacing a faulty electric hydraulic pump

• Repairing a pump control module

• Updating or repairing TCM software where applicable

• Replacing the TCM if it is conclusively diagnosed as faulty

After the repair, the complete circuit should be tested to confirm that the hydraulic pump can be commanded and operates correctly.

Final Thoughts on P0945

P0945 – Hydraulic Pump Relay/Control Circuit/Open indicates that the transmission control system has detected an open or interrupted electrical circuit associated with the hydraulic pump relay or its control system.

The code does not automatically mean that the hydraulic pump is defective.

The most important diagnostic areas are the fuse, hydraulic pump relay, wiring, connectors, terminals, grounds, power supply and pump electrical circuit.

Because the hydraulic pump can be essential to transmission operation, P0945 can become a serious fault if the open circuit prevents adequate hydraulic pressure from being generated.

The correct approach is to locate the electrical interruption before replacing expensive transmission components.

A broken wire, loose connector, corroded terminal or failed relay can produce P0945 and may be considerably simpler to repair than a hydraulic pump or transmission failure.

A systematic circuit diagnosis can therefore prevent unnecessary parts replacement and help protect the transmission from secondary damage.