P0849 Transmission Fluid Pressure Sensor / Switch "B" Circuit Intermittent

The P0849 trouble code is an OBD-II diagnostic trouble code related to the Transmission Fluid Pressure Sensor/Switch "B" circuit.

P0849 – Transmission Fluid Pressure Sensor / Switch "B" Circuit Intermittent

This code means that the PCM or TCM has detected an intermittent or unstable electrical signal from the transmission fluid pressure sensor or switch "B" circuit.

The sensor or switch provides information about hydraulic pressure inside the transmission. The TCM uses this information to control shift timing, clutch engagement and hydraulic pressure.

When the signal repeatedly disappears, changes unexpectedly, or becomes inconsistent with the expected pressure conditions, the control module can store P0849.

What Does P0849 Mean?

The transmission uses hydraulic pressure to operate its internal clutches and bands.

A simplified system is:

Transmission hydraulic pressure → Pressure sensor/switch "B" → Electrical signal → TCM

The TCM continuously monitors this signal.

If the signal is normal for a period of time but then suddenly changes, disappears, or becomes implausible, the TCM may determine that the circuit is intermittent and store P0849.

Unlike:

• P0847 – Transmission Fluid Pressure Sensor/Switch "B" Circuit Low

• P0848 – Transmission Fluid Pressure Sensor/Switch "B" Circuit High

• P0849 – Transmission Fluid Pressure Sensor/Switch "B" Circuit Intermittent

P0849 specifically points toward an unstable or intermittent signal rather than a continuously low or high signal.

What Is Transmission Fluid Pressure Sensor "B"?

Modern automatic transmissions use hydraulic pressure to control different transmission functions.

A pressure sensor or pressure switch can monitor this hydraulic pressure and send information to the TCM.

Depending on the transmission design, the "B" sensor/switch may monitor:

• A particular hydraulic circuit

• A clutch circuit

• A solenoid-controlled pressure circuit

• Line pressure

• Internal transmission pressure

The exact location and function of Pressure Sensor/Switch "B" are manufacturer-specific.

This is important because there is no universal physical location for the "B" sensor.

Symptoms of P0849

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Harsh shifting

• Delayed shifting

• Erratic shifting

• Transmission slipping

• Delayed engagement

• Incorrect shift timing

• Transmission stuck in a gear

• Limp mode

• Reduced transmission performance

• Fluctuating engine RPM during shifts

• Poor acceleration

• Intermittent transmission problems

The vehicle may operate normally when the signal is stable and exhibit problems only when the fault occurs.

Common Causes of P0849

Faulty transmission fluid pressure sensor/switch

The sensor may have an internal electrical problem that causes its output to intermittently drop out.

Damaged wiring

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

Loose connector

A loose terminal can create intermittent electrical contact.

Corroded connector

Corrosion can interrupt the signal and create fluctuating resistance.

Transmission fluid contamination

Dirty or contaminated fluid can affect pressure-sensor operation and hydraulic performance.

Low transmission fluid level

Low fluid can cause unstable hydraulic pressure and may contribute to abnormal pressure readings.

Faulty pressure control solenoid

A malfunctioning solenoid can create pressure fluctuations that may be interpreted as an abnormal sensor signal.

Internal transmission problem

Worn valves, leaking seals, damaged clutch components or hydraulic problems can cause actual pressure to fluctuate.

Poor electrical ground

A poor sensor or module ground can cause an unstable signal.

Short or open circuit

An intermittent short to ground, short to voltage, or open circuit can cause P0849.

Faulty TCM

A TCM internal fault is possible but should generally be considered after the sensor and wiring have been tested.

Is P0849 a Serious Problem?

P0849 can be serious if it causes abnormal transmission operation.

An intermittent pressure signal can cause the TCM to make incorrect hydraulic-pressure or shifting decisions.

If ignored, continued operation may potentially lead to:

• Increased clutch wear

• Harsh engagement

• Transmission slipping

• Overheating

• Additional transmission faults

However, P0849 does not automatically mean the transmission needs to be rebuilt or replaced.

An intermittent electrical connection can produce exactly the same code.

Can You Drive With P0849?

It is better not to drive the vehicle unnecessarily until the cause is identified.

If the transmission shifts normally and the code occurs only occasionally, a short trip to a repair facility may be possible.

However, stop driving if you experience:

• Severe slipping

• Major shift delays

• Strong banging during gear changes

• Loss of Drive or Reverse

• Transmission overheating

• Limp mode

• Sudden loss of power transfer

P0849 vs. P0847

These codes concern the same pressure sensor/switch circuit but describe different electrical conditions.

P0847 – Circuit Low

The signal is lower than expected.

P0849 – Circuit Intermittent

The signal is unstable, intermittent, or temporarily abnormal.

For example:

P0847 → Signal consistently too low

P0849 → Signal repeatedly appears and disappears or behaves abnormally

P0849 vs. P0848

P0848 – Transmission Fluid Pressure Sensor/Switch "B" Circuit High

The signal is detected as too high.

P0849 – Transmission Fluid Pressure Sensor/Switch "B" Circuit Intermittent

The signal is inconsistent or intermittent.

Therefore:

P0847 → Low

P0848 → High

P0849 → Intermittent

How to Diagnose P0849

Because P0849 is an intermittent code, diagnosis can be more difficult than a constant high- or low-signal fault.

1. Scan all modules

Check the PCM and TCM for additional codes.

Look for:

• Pressure sensor codes

• Solenoid codes

• Gear-ratio codes

• Transmission temperature codes

• Communication codes

• Voltage codes

Additional codes can reveal the underlying problem.

2. Check freeze-frame data

Record:

• Engine RPM

• Vehicle speed

• Transmission gear

• Transmission temperature

• Throttle position

• Pressure sensor value

• Battery voltage

This can help determine when the intermittent fault occurred.

3. Check transmission fluid

Inspect:

• Fluid level

• Fluid condition

• Fluid color

• Burnt smell

• Metal particles

• Contamination

Use the manufacturer's procedure because fluid-level checks vary significantly between transmissions.

4. Inspect the pressure sensor/switch connector

Look for:

• Loose terminals

• Corrosion

• Oil contamination

• Water

• Damaged pins

• Poor connector locking

5. Inspect the wiring

Pay particular attention to areas exposed to:

• Heat

• Vibration

• Movement

• Transmission fluid

• Exhaust components

A partially broken wire can produce an intermittent fault that is difficult to reproduce when the vehicle is stationary.

6. Perform a wiggle test

Monitor the pressure sensor signal while carefully moving the wiring harness.

If the signal suddenly changes, the harness or connector may be responsible.

7. Check power and ground

Verify the sensor's:

• Supply voltage

• Reference voltage, if applicable

• Ground

Perform voltage-drop testing where appropriate.

8. Monitor live data

Watch the Pressure Sensor/Switch "B" data while driving.

Look for:

• Sudden signal drops

• Sudden spikes

• Signal dropout

• Implausible pressure changes

• Values that do not correspond to operating conditions

9. Compare sensor data with actual pressure

If required by the manufacturer, connect a mechanical pressure gauge and compare actual hydraulic pressure with the electronic sensor reading.

This is an important step because it can distinguish between:

Electrical sensor problem

and

Actual hydraulic pressure problem

10. Check the pressure control solenoids

A malfunctioning solenoid can cause real pressure fluctuations.

Check solenoid operation and resistance according to manufacturer specifications.

11. Inspect the valve body

If electrical components test correctly but hydraulic pressure is unstable, the valve body may require inspection.

Possible problems include:

• Sticking valves

• Wear

• Contamination

• Internal leakage

12. Check for internal transmission problems

If actual hydraulic pressure is abnormal, inspect for:

• Internal leaks

• Worn clutch components

• Damaged seals

• Pump problems

• Valve-body problems

13. Check TCM operation

Only after the sensor, wiring, power, ground and hydraulic system have been verified should a TCM fault be seriously considered.

Can Low Transmission Fluid Cause P0849?

Yes, it can contribute to the problem.

Low fluid can cause unstable hydraulic pressure.

However, low fluid does not necessarily mean that the pressure sensor circuit itself is electrically faulty.

The fluid level and the electrical circuit should therefore be checked separately.

Can Dirty Transmission Fluid Cause P0849?

Yes, potentially.

Contaminated fluid can affect:

• Pressure sensors

• Valve bodies

• Solenoids

• Hydraulic passages

If the fluid contains excessive debris or has a strong burnt smell, further transmission inspection may be necessary.

Can Wiring Cause P0849?

Yes.

An intermittent wiring fault is one of the most important possibilities.

A wire can look perfectly normal externally while having an internal break.

Vibration can then cause:

Normal connection → Open connection → Normal connection

This can repeatedly interrupt the pressure sensor signal and trigger P0849.

Can a Bad Pressure Sensor Cause P0849?

Yes.

The pressure sensor may have an internal electrical fault that only appears under certain temperature or vibration conditions.

For this reason, a sensor can sometimes test normally when cold and fail after the transmission reaches operating temperature.

Can P0849 Cause Limp Mode?

Yes.

The TCM may activate a protective strategy if it cannot reliably determine transmission pressure.

Depending on the vehicle, the transmission may:

• Hold one gear

• Limit available gears

• Increase line pressure

• Disable certain shift functions

• Illuminate the transmission warning light

Can P0849 Damage the Transmission?

The code itself does not necessarily indicate mechanical damage.

However, if the pressure signal is incorrect and the transmission is repeatedly operating with abnormal hydraulic pressure, continued driving can increase wear.

The cause should therefore be diagnosed promptly.

Common Diagnostic Mistakes With P0849

Avoid these common mistakes:

• Replacing the pressure sensor immediately

• Replacing the transmission without testing the circuit

• Ignoring wiring

• Ignoring connector corrosion

• Checking only the fluid level

• Assuming every P0849 is an internal transmission failure

• Not checking live data

• Not performing a wiggle test

• Ignoring solenoid operation

• Not comparing electronic pressure data with actual hydraulic pressure

What Is the Correct Repair for P0849?

The correct repair depends on the confirmed cause.

Possible repairs include:

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

• Repairing damaged wiring

• Repairing an intermittent connection

• Replacing damaged connector terminals

• Cleaning corroded connectors

• Repairing sensor power or ground

• Repairing a short or open circuit

• Correcting the transmission fluid level

• Replacing contaminated transmission fluid when appropriate

• Repairing or replacing a pressure-control solenoid

• Servicing the valve body

• Repairing internal hydraulic problems

• Repairing the transmission pump

• Repairing internal leaks

• Reprogramming the TCM when required

• Replacing the TCM if confirmed faulty

After the repair, clear the diagnostic codes and perform a road test.

The technician should verify that the pressure signal remains stable under:

• Cold operation

• Normal operating temperature

• Acceleration

• Gear changes

• Different transmission loads

Final Thoughts on P0849

P0849 – Transmission Fluid Pressure Sensor / Switch "B" Circuit Intermittent indicates that the transmission control system has detected an unstable or intermittent signal from the "B" transmission fluid pressure sensor or switch circuit.

The most important distinction is that P0849 does not automatically mean the hydraulic pressure itself is incorrect. The actual problem may be an intermittent sensor, damaged wiring, loose connector, corrosion, poor ground or another electrical issue.

At the same time, genuine hydraulic problems such as low fluid, a malfunctioning pressure-control solenoid, valve-body wear or internal leakage can also create unstable pressure information.

The best diagnostic approach is therefore to check the fluid condition, sensor, wiring, connectors, power and ground first, then monitor live pressure data and compare it with actual hydraulic pressure when necessary. This prevents an intermittent electrical fault from being mistaken for a major transmission failure.