P0879 Transmission Fluid Pressure Sensor / Switch "D" Circuit Intermittent

The P0879 trouble code is an OBD-II diagnostic trouble code related to the Transmission Fluid Pressure Sensor/Switch "D" circuit.

P0879 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Intermittent

This code means that the transmission control system has detected an intermittent or unstable electrical signal from the Transmission Fluid Pressure Sensor/Switch "D" circuit.

The sensor or switch is used to monitor transmission fluid pressure and provide information to the transmission control module (TCM) or powertrain control module (PCM). The module uses this information to determine whether hydraulic pressure is appropriate for the current transmission operating conditions.

When the signal from the "D" pressure sensor or switch repeatedly becomes unstable, disappears, or changes unexpectedly, the control module can store P0879.

Possible causes include:

• Faulty transmission fluid pressure sensor/switch "D"

• Damaged sensor wiring

• Intermittent open circuit

• Loose electrical connector

• Corroded connector terminals

• Poor terminal contact

• Wiring harness damage

• Short circuit

• Low transmission fluid level

• Incorrect transmission fluid

• Contaminated transmission fluid

• Internal hydraulic pressure problem

• Faulty valve body

• Sticking transmission valve

• Internal transmission problem

• Poor TCM/PCM connection

• Faulty TCM/PCM

The exact location and function of the "D" pressure sensor/switch varies depending on the transmission and vehicle manufacturer.

What Does P0879 Mean?

P0879 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Intermittent

Automatic transmissions use hydraulic pressure to operate clutches, bands, valves and other components.

The pressure sensor/switch provides the transmission controller with information about hydraulic pressure.

A simplified system can be represented as:

Transmission hydraulic system → Pressure sensor/switch "D" → Electrical signal → TCM/PCM

The TCM compares the pressure signal with the expected pressure for the current operating conditions.

If the signal intermittently becomes abnormal, the TCM can store P0879.

For example:

Normal pressure signal → Signal briefly drops out → Signal returns → TCM detects an intermittent condition → P0879 stored

What Does “Intermittent” Mean?

The word “Intermittent” is particularly important in P0879.

It means that the pressure sensor/switch circuit does not necessarily fail continuously.

The signal may be normal for several minutes and then suddenly become abnormal.

The problem may occur when:

• The vehicle hits a bump

• The transmission heats up

• The engine or transmission vibrates

• The wiring moves

• The connector gets wet

• The transmission changes gear

• Electrical loads change

• Fluid temperature increases

• The sensor reaches operating temperature

This is why P0879 can sometimes be difficult to diagnose.

Symptoms of P0879

The symptoms can vary significantly depending on the vehicle and transmission.

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Harsh shifting

• Delayed shifting

• Erratic shifting

• Transmission slipping

• Gear changes at incorrect RPM

• Transmission stuck in one gear

• Limp mode

• Delayed engagement

• Loss of certain gears

• Reduced transmission performance

• Intermittent transmission operation

Some vehicles may store P0879 while continuing to operate almost normally.

Common Causes of P0879

Faulty transmission fluid pressure sensor/switch "D"

The sensor itself may have an internal electrical fault that causes the output signal to intermittently drop out or become unstable.

Damaged wiring

A partially broken wire can make and lose contact as the transmission or wiring harness moves.

Loose connector

A connector that is not securely locked can produce intermittent signal interruptions.

Corroded terminals

Corrosion can increase resistance and cause an unstable pressure signal.

Poor terminal tension

A terminal may appear connected but fail to maintain reliable contact.

Short circuit

Damaged insulation can allow the sensor circuit to contact another circuit.

Low transmission fluid

Insufficient fluid can cause abnormal hydraulic pressure and may contribute to pressure-related faults.

However, low fluid does not automatically mean the electrical sensor circuit itself is defective.

Incorrect transmission fluid

The wrong fluid can affect transmission hydraulic behavior and shift quality.

Contaminated fluid

Dirty or degraded fluid can affect valves, pressure regulation and hydraulic operation.

Valve body problems

A sticking valve or hydraulic control problem can produce pressure readings that do not match expected values.

Internal transmission problems

Worn clutches, seals or other hydraulic components can affect actual transmission pressure.

Faulty TCM/PCM

A control-module problem is possible but should generally be investigated after the sensor and wiring have been tested.

Is P0879 a Serious Transmission Problem?

P0879 should be taken seriously, especially if transmission symptoms are present.

Transmission fluid pressure is directly related to how the transmission applies its clutches and performs gear changes.

If the pressure signal is unreliable, the TCM may not be able to accurately control transmission operation.

Continued operation with a genuine hydraulic pressure problem can potentially contribute to transmission damage.

However, P0879 does not automatically mean that the transmission needs to be rebuilt.

An intermittent connector or damaged wire can produce exactly the same code.

Can You Drive With P0879?

It is best to avoid unnecessary driving until the cause is identified, particularly if the vehicle is shifting abnormally.

Driving may be possible for a short distance if the vehicle operates normally, but the code should not be ignored.

Stop driving if you experience:

• Severe slipping

• Harsh engagement

• Transmission stuck in one gear

• Loss of reverse

• Loss of drive

• Significant fluid leakage

• Burning transmission-fluid smell

• Severe or unpredictable shifting

P0879 vs. P0878

P0878 – Transmission Fluid Pressure Sensor/Switch "D" Circuit High

The TCM detects a higher-than-expected electrical signal from the "D" pressure sensor/switch circuit.

P0879 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Intermittent

The signal intermittently becomes abnormal or unstable.

In simple terms:

P0878 → Signal high

P0879 → Signal intermittent

P0879 vs. P0877

P0877 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Low

The TCM detects a lower-than-expected signal.

P0879 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Intermittent

The signal is unstable or intermittently lost.

A wiring fault can sometimes cause either condition depending on how the circuit fails.

P0879 vs. P0876

P0876 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Range/Performance

The pressure sensor signal is outside the expected range or does not correspond correctly to operating conditions.

P0879 – Circuit Intermittent

The electrical signal becomes unstable or is intermittently interrupted.

The distinction is important because P0876 may indicate a signal plausibility problem, while P0879 specifically points toward an intermittent condition.

P0879 vs. P0875

P0875 – Transmission Fluid Pressure Sensor/Switch "D" Circuit

This is the general circuit fault for the "D" pressure sensor/switch.

P0879 indicates that the circuit problem is specifically intermittent.

How to Diagnose P0879

Because P0879 is an intermittent code, the diagnosis should focus on both the electrical circuit and the actual transmission pressure.

1. Scan the vehicle

Check all modules for:

• Stored codes

• Pending codes

• History codes

• Transmission-related codes

• Communication codes

Other codes may help identify the root cause.

2. Check freeze-frame data

Look at:

• Vehicle speed

• Engine RPM

• Transmission temperature

• Gear commanded

• Gear actually selected

• Pressure sensor voltage

• Throttle position

• Battery voltage

This can reveal the conditions under which the intermittent fault occurred.

3. Check the transmission fluid

Inspect:

• Fluid level

• Fluid condition

• Fluid color

• Fluid smell

• Signs of contamination

• Metal particles

If the fluid level is low, determine why before simply adding fluid.

4. Inspect for leaks

Check around:

• Transmission pan

• Transmission cooler lines

• Seals

• Electrical connectors

• Axle seals

• Valve-body area

A low fluid level caused by a leak can create additional transmission problems.

5. Locate pressure sensor/switch "D"

Use the vehicle-specific service information to identify the exact sensor.

The designation "D" is not necessarily located in the same place on every vehicle.

It may be:

• Inside the transmission

• Mounted on the valve body

• Integrated into a transmission control assembly

• Installed in an external hydraulic circuit

6. Inspect the connector

Look for:

• Loose connection

• Corrosion

• Bent terminals

• Oil contamination

• Water contamination

• Damaged locking mechanism

7. Inspect the wiring harness

Look for:

• Broken wires

• Chafing

• Pinched wiring

• Heat damage

• Oil contamination

• Previous repairs

Transmission wiring can be exposed to significant heat and vibration.

8. Perform a wiggle test

Monitor the sensor signal while carefully moving the wiring harness and connector.

If the signal changes suddenly, an intermittent wiring or connector problem may be present.

9. Check sensor reference voltage

Many pressure sensors use a reference voltage supplied by the control module.

Verify that the reference voltage remains stable.

10. Check sensor ground

A poor sensor ground can cause an intermittent or unstable output signal.

Perform a voltage-drop test where appropriate.

11. Monitor the sensor signal

Use a scan tool to monitor the pressure sensor value while:

• Idling

• Selecting gears

• Accelerating

• Decelerating

• Changing transmission temperature

An unstable signal can help confirm an electrical problem.

12. Compare commanded and actual pressure

Where the scan tool provides these values, compare the expected pressure with the sensor reading.

A mismatch may indicate either:

• Electrical sensor problems

• Hydraulic pressure problems

13. Test the sensor

Follow the manufacturer's testing procedure.

Check:

• Resistance where applicable

• Reference voltage

• Ground

• Signal voltage

• Signal continuity

Do not use a generic resistance specification unless it applies to that specific sensor.

14. Perform a hydraulic pressure test

If the electrical circuit appears normal but the pressure values remain abnormal, a mechanical pressure test may be necessary.

A pressure gauge can determine whether the transmission is actually producing the expected hydraulic pressure.

15. Inspect the valve body if necessary

If actual hydraulic pressure is abnormal, inspect for:

• Sticking valves

• Wear

• Blockages

• Contamination

• Solenoid problems

16. Evaluate the TCM/PCM

Only after the sensor, wiring, power, ground and hydraulic system have been properly checked should the control module be considered.

Why P0879 Can Be Difficult to Find

Intermittent faults are often difficult because the circuit may test perfectly when the vehicle is stationary.

For example:

Loose connector → Normal signal in workshop → Vehicle vibrates → Signal briefly disappears → P0879

Another possibility is:

Sensor works when cold → Transmission heats up → Sensor develops internal fault → Signal becomes unstable → P0879

Therefore, a simple visual inspection may not be enough.

Can Low Transmission Fluid Cause P0879?

Low transmission fluid can contribute to pressure-related problems, but it does not automatically explain an intermittent electrical circuit fault.

If the fluid is low, determine the cause of the loss.

A low fluid level can cause:

• Low hydraulic pressure

• Delayed engagement

• Slipping

• Erratic shifting

• Transmission overheating

The sensor circuit should still be electrically tested.

Can Dirty Transmission Fluid Cause P0879?

Yes, it can contribute indirectly.

Contaminated fluid can affect:

• Valve-body operation

• Hydraulic pressure

• Pressure-control components

• Sensor operation

However, contaminated fluid does not necessarily mean the pressure sensor itself is defective.

Can a Bad Pressure Sensor Cause P0879?

Yes.

A failing pressure sensor/switch "D" is one of the most direct causes.

The sensor may have an internal fault that occurs only:

• At certain temperatures

• During vibration

• Under certain pressure conditions

• During specific gear changes

Can Wiring Cause P0879?

Yes.

Because P0879 specifically describes an intermittent circuit condition, wiring and connector problems should be considered carefully.

A partially broken wire or loose terminal can cause the signal to disappear momentarily.

Can P0879 Cause Limp Mode?

Yes.

If the TCM cannot reliably determine transmission pressure, it may activate a protective strategy.

The transmission may then:

• Limit available gears

• Hold one gear

• Increase line pressure

• Change shift strategy

• Enter limp mode

The exact response depends on the vehicle.

Can P0879 Damage the Transmission?

Potentially, yes, if the underlying problem is hydraulic or causes incorrect clutch pressure.

If the transmission is slipping or operating with incorrect hydraulic pressure, continued driving can increase wear.

However, an intermittent electrical fault caused by a connector or wire does not necessarily mean the transmission itself is mechanically damaged.

Common Diagnostic Mistakes With P0879

Common mistakes include:

• Replacing the pressure sensor immediately

• Rebuilding the transmission without testing the circuit

• Ignoring the wiring

• Ignoring the connector

• Ignoring transmission fluid level

• Assuming the code automatically means low hydraulic pressure

• Not checking freeze-frame data

• Not performing a wiggle test

• Not monitoring live sensor data

• Not checking sensor reference voltage

• Not checking sensor ground

• Skipping hydraulic pressure testing when necessary

• Replacing the TCM without confirming a module fault

What Is the Correct Repair for P0879?

The correct repair depends on the actual cause.

Possible repairs include:

• Replacing the transmission fluid pressure sensor/switch "D"

• Repairing damaged wiring

• Repairing an intermittent open circuit

• Repairing a shorted wire

• Replacing damaged connector terminals

• Cleaning corroded connectors

• Securing a loose connector

• Repairing the transmission harness

• Correcting transmission fluid level

• Repairing a transmission fluid leak

• Replacing contaminated or incorrect fluid where appropriate

• Repairing a valve-body problem

• Replacing a faulty pressure-control component

• Repairing an internal hydraulic problem

• Replacing the TCM/PCM if confirmed faulty

After the repair, clear the diagnostic code and perform a road test while monitoring the pressure sensor signal and transmission operation.

Final Thoughts on P0879

P0879 – Transmission Fluid Pressure Sensor/Switch "D" Circuit Intermittent means that the transmission control system has detected an unstable or intermittently interrupted signal from the "D" transmission fluid pressure sensor/switch circuit.

The most important areas to inspect are the pressure sensor, wiring harness, electrical connector, reference voltage, sensor ground, transmission fluid condition and actual hydraulic pressure.

The key point is that P0879 does not automatically mean that the transmission needs to be rebuilt. A loose connector, corroded terminal or partially broken wire can cause the exact same code.

Because the fault is intermittent, monitoring live data, performing a wiggle test and checking the circuit under real operating conditions can be much more useful than a simple visual inspection.