P0842 Transmission Fluid Pressure Sensor / Switch "A" Circuit Low

The P0842 – Transmission Fluid Pressure Sensor/Switch "A" Circuit Low diagnostic trouble code (DTC) means that the transmission control module (TCM) has detected an electrical signal from the transmission fluid pressure sensor or switch "A" that is lower than the expected value.

The transmission uses hydraulic pressure to control gear changes, clutch engagement, and other internal functions. The pressure sensor or switch provides feedback to the TCM, allowing it to determine whether the transmission is producing the expected hydraulic pressure.

An important point is that P0842 describes a low electrical circuit signal. It does not automatically mean that the actual hydraulic pressure inside the transmission is low.

What Does P0842 Mean?

The code can be understood as:

• P – Powertrain

• 08 – Transmission control system

• 42 – Specific pressure-sensor circuit fault

• Transmission Fluid Pressure Sensor/Switch "A" – The designated pressure-monitoring circuit

• Circuit Low – The electrical signal is below the expected level

In simple terms:

P0842 means the TCM is seeing a lower-than-expected electrical signal from transmission fluid pressure sensor/switch "A".

The exact sensor, switch, wiring configuration, and voltage range depend on the transmission manufacturer.

What Is a Transmission Fluid Pressure Sensor?

Automatic transmissions depend on hydraulic pressure to operate correctly.

Transmission fluid pressure is used to control:

• Clutch packs

• Brake elements

• Shift valves

• Solenoids

• Gear engagement

• Hydraulic circuits

A pressure sensor converts hydraulic pressure into an electrical signal.

The TCM then uses this signal to determine whether actual pressure corresponds to the commanded operating condition.

What Is Pressure Switch "A"?

Some transmissions use a pressure switch instead of a variable pressure sensor.

A pressure switch generally changes its electrical state when hydraulic pressure reaches a predetermined threshold.

Depending on the transmission, pressure switch "A" may be:

• Mounted directly in the valve body

• Installed in a hydraulic passage

• Integrated into a transmission control assembly

• Part of a mechatronic unit

• Combined with other pressure switches

The designation "A" is manufacturer-specific, so its location and function vary between vehicles.

What Does "Circuit Low" Mean?

The term "Circuit Low" primarily refers to the electrical signal.

It does not necessarily mean that hydraulic pressure is low.

For example, the TCM may detect a low voltage because of:

• Short to ground

• Faulty pressure sensor

• Faulty pressure switch

• Damaged wiring

• Poor connector

• Low reference voltage

• Sensor-ground problem

Actual low hydraulic pressure can also create a low sensor reading on some systems, but this must be confirmed separately.

P0842 vs. P0841, P0843 and P0844

The related codes help distinguish different problems with pressure sensor/switch "A".

P0841 – Range/Performance

The pressure-sensor signal does not behave as expected.

P0842 – Circuit Low

The electrical signal is lower than expected.

P0843 – Circuit High

The electrical signal is higher than expected.

P0844 – Circuit Intermittent

The signal is unstable or intermittently lost.

Therefore, P0842 specifically points toward a low electrical signal condition.

Symptoms of P0842

The symptoms depend on the vehicle and the transmission design.

Possible symptoms include:

Check Engine Light

The malfunction indicator lamp may illuminate.

Transmission warning light

Some vehicles may display a transmission or drivetrain warning.

Harsh shifting

Gear changes may become harder than normal.

Delayed shifting

The transmission may hesitate during gear changes.

Erratic shifting

Shift behavior may become unpredictable.

Delayed Drive or Reverse engagement

The transmission may take longer to engage.

Transmission slipping

If actual hydraulic pressure is insufficient, clutch engagement can be affected.

Limp mode

The TCM may activate a fail-safe strategy.

Transmission stuck in one gear

Some vehicles may select a default gear.

Reduced acceleration

The transmission may limit its operation to protect itself.

Is P0842 a Serious Problem?

P0842 can be serious, especially if transmission symptoms are present.

If the problem is caused by a sensor, connector, or wiring fault, the repair may be relatively simple.

However, if the low electrical signal accurately reflects low hydraulic pressure, continued driving can cause:

• Clutch slipping

• Excessive heat

• Increased wear

• Poor lubrication

• Further transmission damage

The vehicle should be inspected promptly if P0842 occurs together with slipping, severe shifting, overheating, or loss of gears.

Common Causes of P0842

Several problems can cause this DTC.

Faulty transmission fluid pressure sensor "A"

The sensor may internally fail and produce a low signal.

Faulty pressure switch "A"

The switch may remain in the wrong electrical state.

Short to ground

A signal wire may be contacting ground.

Damaged wiring

The harness may have:

• Broken conductors

• Chafed insulation

• Corrosion

• High resistance

• Poor previous repairs

Poor connector

Loose or corroded terminals can reduce the sensor signal.

Low reference voltage

The sensor may not receive the required reference voltage.

Poor sensor ground

An abnormal ground circuit can affect sensor output.

Low transmission fluid

Insufficient fluid can result in inadequate hydraulic pressure.

Restricted transmission filter

A restricted filter can reduce fluid flow.

Weak transmission pump

A worn pump may fail to generate sufficient pressure.

Pressure-control solenoid problem

A malfunctioning solenoid can cause incorrect hydraulic pressure.

Valve-body problem

Sticking valves or internal wear can affect pressure.

Internal hydraulic leakage

Worn seals or internal components can prevent pressure from reaching the required level.

Can a Bad Pressure Sensor Cause P0842?

Yes.

A defective pressure sensor can send a voltage that is lower than expected even when actual transmission pressure is normal.

This is why the sensor should not automatically be blamed without testing the circuit and comparing sensor information with actual hydraulic pressure.

Can Wiring Cause P0842?

Yes.

A short to ground is a particularly important possibility.

If the sensor signal wire is unintentionally grounded, the TCM may see an extremely low voltage.

Inspect the wiring for:

• Chafing

• Pinching

• Broken insulation

• Contact with the transmission housing

• Poor aftermarket repairs

• Corrosion

Can a Connector Cause P0842?

Yes.

A poor connector can create excessive resistance or an unstable circuit.

Inspect for:

• Corroded terminals

• Loose pins

• Bent terminals

• Water intrusion

• Transmission-fluid contamination

• Damaged seals

• Broken locking mechanisms

Can Low Transmission Fluid Cause P0842?

Yes.

Low fluid can cause genuine hydraulic-pressure problems.

The symptoms may become more noticeable during:

• Acceleration

• Cornering

• Braking

• High engine RPM

• Gear changes

If fluid level is low, determine why it is low rather than simply adding fluid.

A leak, incorrect service procedure, or internal problem may be responsible.

Can Dirty Transmission Fluid Cause P0842?

Yes, potentially.

Contaminated fluid can affect:

• Pressure sensors

• Pressure switches

• Solenoids

• Valve-body operation

• Hydraulic passages

If the fluid contains excessive clutch material or metal particles, further transmission inspection may be necessary.

Can a Transmission Pump Cause P0842?

Yes.

The transmission pump provides the hydraulic pressure required for transmission operation.

A worn pump can cause:

• Low line pressure

• Delayed engagement

• Slipping

• Poor shifting

• Pressure-related fault codes

Actual hydraulic pressure should be measured before condemning the pump.

Can a Valve Body Cause P0842?

Yes.

The valve body directs transmission fluid through different hydraulic circuits.

A valve-body problem can cause:

• Low pressure

• Pressure leakage

• Sticking valves

• Incorrect pressure regulation

• Delayed shifts

If the electrical circuit is operating correctly, the hydraulic system should be investigated.

Can a Pressure-Control Solenoid Cause P0842?

Yes.

A pressure-control solenoid regulates hydraulic pressure.

If it sticks, becomes restricted, or fails electrically, pressure may not reach the expected level.

This can result in transmission symptoms and pressure-related diagnostic codes.

Can P0842 Cause Limp Mode?

Yes.

The TCM may activate a fail-safe strategy when it detects unreliable pressure information.

Depending on the vehicle, this can result in:

• Fixed gear operation

• Limited gear selection

• Harsh shifting

• Disabled overdrive

• Transmission warning message

Can P0842 Cause Transmission Slipping?

Yes, potentially.

If the low circuit signal is caused by actual low hydraulic pressure, clutch elements may not receive enough pressure to engage properly.

This can cause slipping.

However, if P0842 is caused only by a faulty electrical signal, actual hydraulic pressure may still be normal.

Can P0842 Cause Transmission Overheating?

Yes, indirectly.

Low hydraulic pressure can cause clutch slipping.

Slipping generates heat, and excessive heat can damage transmission fluid and internal components.

If P0842 is accompanied by slipping, continued driving should be avoided until the problem is diagnosed.

How Is P0842 Diagnosed?

A proper diagnosis should determine whether the low signal is caused by an electrical fault or a genuine hydraulic-pressure problem.

1. Scan all modules

Check for additional codes involving:

• Pressure sensors

• Solenoids

• Gear ratios

• Transmission temperature

• TCM communication

2. Check freeze-frame data

Review:

• Engine RPM

• Vehicle speed

• Transmission temperature

• Selected gear

• Throttle position

• Battery voltage

3. Check transmission fluid

Verify the correct fluid level and condition using the manufacturer's procedure.

4. Inspect the pressure sensor

Check for physical damage and contamination.

5. Inspect the connector

Look for corrosion, loose terminals, and fluid intrusion.

6. Inspect the wiring

Look for shorts to ground, broken wires, and high resistance.

7. Verify reference voltage

Confirm that the sensor receives the specified reference voltage.

8. Verify the ground

Make sure the sensor ground circuit is stable.

9. Check the sensor signal

Compare the measured signal with manufacturer specifications.

10. Monitor live data

Observe the pressure signal during different transmission operating conditions.

11. Measure hydraulic pressure

If the electrical circuit checks out, verify actual transmission pressure.

How to Test P0842 With a Multimeter

The exact procedure varies by vehicle.

Use the manufacturer's wiring diagram and specifications.

Check the reference voltage.

Verify that the sensor receives the correct reference supply.

Check the ground.

Confirm that the ground circuit is reliable.

Check signal voltage.

Determine whether the signal is below the specified operating range.

Check for a short to ground.

Inspect the signal circuit for unwanted contact with ground.

Check continuity.

Verify the circuit between the pressure sensor and TCM.

Check resistance.

Excessive resistance in the circuit can cause incorrect readings.

What Voltage Should the Pressure Sensor Have?

There is no universal voltage value for P0842.

Different manufacturers use different pressure sensors and electrical systems.

The sensor output may change according to:

• Hydraulic pressure

• Transmission temperature

• Selected gear

• Engine load

• Solenoid operation

Always use the vehicle-specific technical specifications rather than a generic voltage value.

How Can You Tell If P0842 Is Electrical or Hydraulic?

This is one of the most important diagnostic questions.

If the sensor signal is low but actual hydraulic pressure is normal, investigate:

• Pressure sensor

• Pressure switch

• Signal wiring

• Short to ground

• Reference voltage

• Ground

• Connector

If the electrical circuit is correct and actual hydraulic pressure is genuinely low, investigate:

• Transmission fluid level

• Filter

• Pump

• Pressure-control solenoid

• Valve body

• Internal hydraulic leakage

A hydraulic pressure test is often the best way to distinguish between the two.

What If P0842 Appears Only When the Transmission Is Hot?

A temperature-dependent fault can point toward:

• Pressure sensor failure

• Wiring resistance

• Connector problems

• Solenoid problems

• Hydraulic leakage

• Low fluid level

Monitor the sensor signal as the transmission reaches operating temperature.

If the signal suddenly falls when the transmission becomes hot, this can provide an important diagnostic clue.

What If P0842 Appears Only During Gear Changes?

A pressure-related problem that occurs during a particular shift may indicate an issue with the hydraulic circuit involved in that shift.

Possible causes include:

• Pressure sensor

• Pressure switch

• Shift solenoid

• Pressure-control solenoid

• Valve body

• Wiring

• Low hydraulic pressure

Recording transmission live data during the affected shift can help identify the cause.

Can P0842 Be Intermittent?

Yes.

Although P0842 describes a low-signal condition, the underlying problem can still be intermittent.

For example, a damaged wire may occasionally touch ground.

A loose connector can also cause the signal to drop under:

• Vibration

• Engine movement

• Transmission movement

• Heat

• Rough-road conditions

If the fault disappears and returns, inspect the harness and connectors carefully.

What If P0842 Appears With P0841?

P0841 indicates a range/performance problem, while P0842 specifically indicates a low circuit signal.

If both codes are present, investigate the complete pressure sensor "A" circuit rather than replacing parts immediately.

The combination can point toward a sensor or circuit problem, although actual hydraulic issues should also be ruled out.

What If P0842 Appears With P0843?

P0842 indicates a low signal, while P0843 indicates a high signal.

If both appear at different times, an unstable electrical circuit, wiring problem, connector issue, or sensor fault may be worth investigating.

A single component can sometimes produce different readings under different conditions.

Common Diagnostic Mistakes

Replacing the pressure sensor immediately

A short to ground or wiring fault can produce P0842.

Assuming low circuit means low fluid pressure

The DTC refers primarily to the electrical signal.

Ignoring the transmission fluid

Actual low hydraulic pressure remains a possibility.

Ignoring the connector

Loose or corroded terminals can cause low sensor signals.

Replacing the transmission

P0842 does not automatically mean internal transmission failure.

Using generic sensor specifications

Pressure sensors vary between manufacturers.

Skipping hydraulic pressure testing

If the electrical system is correct, actual pressure must be investigated.

How Is P0842 Repaired?

The correct repair depends on the confirmed cause.

Possible repairs include:

• Repairing a short to ground

• Repairing damaged wiring

• Replacing damaged terminals

• Repairing or replacing the connector

• Cleaning corrosion

• Replacing the pressure sensor

• Replacing the pressure switch

• Repairing the sensor ground

• Restoring reference voltage

• Correcting transmission-fluid level

• Replacing contaminated or incorrect fluid where appropriate

• Replacing a restricted filter where applicable

• Repairing a pressure-control solenoid

• Servicing the valve body

• Repairing hydraulic leaks

• Repairing the transmission pump

• Repairing confirmed internal transmission damage

Does P0842 Mean the Transmission Needs to Be Rebuilt?

No.

P0842 alone is not proof that the transmission needs rebuilding.

The cause could be something as simple as:

• A damaged wire

• A short to ground

• A corroded connector

• A faulty pressure sensor

Before considering major transmission work, the complete electrical circuit and actual hydraulic pressure should be verified.

Can You Drive With P0842?

It depends on the symptoms.

If the vehicle shifts normally and the code has appeared without noticeable transmission problems, a short trip to a repair facility may be possible.

However, continued driving is not advisable if you experience:

• Transmission slipping

• Severe shift shock

• Delayed engagement

• Loss of gears

• Transmission overheating

• Burning fluid smell

• Limp mode

If actual hydraulic pressure is low, continued driving can cause additional internal wear.

How to Reset P0842

Once the underlying fault has been repaired, clear the DTC with a suitable diagnostic scanner.

A proper procedure is:

Diagnose → Repair → Verify sensor circuit → Verify hydraulic pressure if necessary → Road test → Clear DTC → Rescan

If P0842 returns, the original problem has not been fully resolved.

Disconnecting the battery is not a proper repair and may erase learned transmission adaptations on some vehicles.

Final Thoughts on P0842

P0842 – Transmission Fluid Pressure Sensor/Switch "A" Circuit Low indicates that the TCM has detected an electrical signal lower than expected from transmission fluid pressure sensor or switch "A".

Possible causes include a faulty pressure sensor or switch, short to ground, damaged wiring, poor connector, incorrect reference voltage, poor ground, low transmission fluid, restricted fluid flow, pressure-control solenoid problems, valve-body faults, transmission-pump problems, or internal hydraulic leakage.

The most important point is that P0842 does not automatically mean that transmission hydraulic pressure is low.

The diagnostic process should separate the electrical system from the hydraulic system:

Electrical side:
Pressure Sensor/Switch "A" → Reference Voltage → Ground → Signal → Wiring → Connector → TCM

Hydraulic side:
Transmission Fluid → Filter → Pump → Solenoids → Valve Body → Hydraulic Circuits

If the sensor circuit produces a low signal while actual hydraulic pressure is normal, the sensor or electrical circuit is the likely source. If the electrical system is functioning correctly and hydraulic pressure is genuinely low, the transmission's hydraulic system needs further investigation.

Therefore, do not replace the entire transmission based on P0842 alone. A systematic diagnosis can often identify a much simpler electrical or sensor-related fault.