P0845 Transmission Fluid Pressure Sensor / Switch "B" Circuit

The P0845 trouble code is an OBD-II diagnostic trouble code related to the Transmission Fluid Pressure Sensor/Switch "B" circuit.

P0845 – Transmission Fluid Pressure Sensor / Switch "B" Circuit

This code means that the transmission control system has detected a problem with the electrical circuit associated with the "B" transmission fluid pressure sensor or pressure switch.

The sensor or switch provides the PCM or TCM with information about hydraulic pressure inside the transmission. This information can be used to control gear changes, clutch engagement, line pressure and transmission protection strategies.

P0845 is a relatively general circuit code. Unlike P0846 (range/performance), P0847 (low), P0848 (high), and P0849 (intermittent), P0845 does not necessarily identify the exact electrical condition. It indicates that the control system has detected a problem with the Pressure Sensor/Switch "B" circuit.

What Does P0845 Mean?

Automatic transmissions depend on hydraulic pressure to engage clutches and control different gears.

The pressure sensor/switch converts hydraulic pressure or pressure conditions into an electrical signal that the transmission control module can monitor.

A simplified system is:

Transmission Hydraulic Circuit → Pressure Sensor/Switch "B" → Electrical Signal → TCM

When the TCM detects that this circuit is not operating as expected, it can store P0845.

The fault can be electrical, sensor-related, or, depending on the vehicle, associated with an underlying hydraulic problem.

What Is Transmission Fluid Pressure Sensor/Switch "B"?

Modern automatic transmissions may use multiple pressure sensors or switches.

The letter "B" identifies a particular pressure-monitoring circuit, but it does not mean the same physical sensor on every vehicle.

Depending on the transmission, Sensor/Switch "B" may monitor:

• A specific clutch circuit

• A hydraulic passage

• Line pressure

• A solenoid-controlled pressure circuit

• A particular transmission gear circuit

Because manufacturers use different transmission designs, the location and exact function of Pressure Sensor/Switch "B" must be determined from vehicle-specific service information.

Symptoms of P0845

The symptoms can vary considerably depending on the vehicle and the nature of the circuit fault.

Possible symptoms include:

• Check Engine Light

• Transmission warning light

• Harsh shifting

• Delayed gear changes

• Erratic shifting

• Transmission slipping

• Delayed Drive engagement

• Delayed Reverse engagement

• Transmission stuck in a gear

• Limp mode

• Reduced transmission performance

• Incorrect shift timing

• Fluctuating engine RPM during shifts

• Poor acceleration

• Increased transmission temperature

• Intermittent transmission operation

Some vehicles may store P0845 while continuing to drive almost normally.

Common Causes of P0845

Faulty transmission fluid pressure sensor/switch "B"

The sensor or switch can fail internally and produce an incorrect signal.

Damaged wiring

The wiring between the pressure sensor and TCM can be damaged by:

• Heat

• Vibration

• Chafing

• Pinching

• Transmission fluid

• Corrosion

• Previous repairs

Corroded connector

Moisture or transmission fluid contamination can cause corrosion and interfere with the electrical connection.

Loose terminal

A terminal that does not make proper contact can cause an abnormal sensor signal.

Open circuit

A broken wire or disconnected connector can interrupt the pressure-sensor circuit.

Short to ground

A signal wire contacting ground can alter the sensor signal.

Short to voltage

A signal wire contacting battery or ignition voltage can produce an abnormal input.

Poor sensor ground

A poor ground can cause incorrect pressure-sensor readings.

Incorrect reference voltage

Some pressure sensors use a reference voltage supplied by the control system. An abnormal reference voltage can affect the sensor signal.

Low transmission fluid

Low fluid can cause hydraulic pressure problems and abnormal transmission operation.

Contaminated transmission fluid

Dirty or degraded fluid can affect pressure sensors, solenoids and valve-body operation.

Faulty pressure-control solenoid

A solenoid problem can produce abnormal hydraulic pressure.

Valve-body problems

Sticking valves, wear or contamination can cause hydraulic-pressure problems.

Internal transmission problem

Worn clutches, seals, pumps or internal hydraulic passages can affect pressure.

Faulty TCM

A TCM problem is possible, although the sensor and wiring should be tested first.

Is P0845 a Serious Problem?

P0845 can be a serious transmission-related fault, particularly if the vehicle has shifting problems.

The pressure information supplied by the sensor can be important for controlling:

• Clutch engagement

• Gear changes

• Hydraulic pressure

• Torque management

If the TCM cannot reliably obtain this information, it may activate a protective strategy.

However, P0845 does not automatically mean that the transmission has suffered internal mechanical damage.

A faulty sensor, connector or wiring problem can trigger the code.

Can You Drive With P0845?

If the transmission is shifting normally, the vehicle may be driven a short distance to a repair facility.

However, continued driving is not recommended until the problem has been diagnosed.

Stop driving if you notice:

• Severe transmission slipping

• Violent shifting

• Loss of Drive

• Loss of Reverse

• Major engagement delays

• Transmission overheating

• Limp mode

• Unusual transmission noises

A pressure-sensor circuit fault can sometimes develop into a more significant transmission-control problem.

P0845 vs. P0846

These codes are closely related.

P0845 – Transmission Fluid Pressure Sensor/Switch "B" Circuit

A general problem has been detected in the circuit.

P0846 – Transmission Fluid Pressure Sensor/Switch "B" Circuit Range/Performance

The signal is present but does not correspond correctly to the expected range or operating conditions.

In simple terms:

P0845 → General circuit problem

P0846 → Signal range/performance problem

P0845 vs. P0847

P0847 – Circuit Low

The pressure sensor/switch "B" signal is lower than expected.

P0845 – Circuit

A problem has been detected, but the code does not specifically classify it as low.

Therefore:

P0845 → General circuit fault

P0847 → Low signal

P0845 vs. P0848

P0848 – Circuit High

The signal from Pressure Sensor/Switch "B" is higher than expected.

P0845 is a more general circuit code.

So:

P0845 → General circuit problem

P0848 → High signal

P0845 vs. P0849

P0849 – Circuit Intermittent

The pressure sensor/switch "B" signal is intermittently abnormal.

P0845 indicates a general problem with the circuit without specifically identifying the fault as intermittent.

The codes can sometimes appear together if the same circuit develops different electrical conditions.

How to Diagnose P0845

A proper diagnosis should investigate both the electrical circuit and the hydraulic system.

1. Scan all control modules

Check the PCM and TCM for additional codes.

Pay attention to:

• Pressure sensor codes

• Solenoid codes

• Gear-ratio codes

• Transmission temperature codes

• Communication codes

• Voltage-related codes

Additional codes can provide important clues.

2. Check freeze-frame data

Record information such as:

• Engine RPM

• Vehicle speed

• Transmission gear

• Transmission temperature

• Throttle position

• Pressure sensor value

• Battery voltage

This helps determine the conditions under which P0845 was stored.

3. Check transmission fluid

Inspect:

• Fluid level

• Fluid condition

• Fluid color

• Burnt smell

• Metal particles

• Contamination

Always follow the manufacturer's fluid-checking procedure.

4. Locate Pressure Sensor/Switch "B"

Use the vehicle-specific wiring diagram and transmission service information.

Do not assume that the sensor has the same location on different vehicles.

5. Inspect the sensor and connector

Check for:

• Corrosion

• Bent pins

• Loose terminals

• Water

• Transmission-fluid contamination

• Damaged connector locking mechanism

6. Inspect the wiring

Look for:

• Broken wires

• Chafing

• Melted insulation

• Pinched wires

• Oil-soaked wiring

• Previous repairs

7. Test sensor power and reference voltage

Depending on the sensor design, check:

• Supply voltage

• 5-volt reference

• Ground

• Signal voltage

Compare the readings with manufacturer specifications.

8. Check for shorts and open circuits

Test the circuit for:

• Short to ground

• Short to voltage

• Open circuit

• Excessive resistance

9. Check the sensor ground

A poor ground can produce an incorrect sensor signal.

A voltage-drop test can help identify excessive resistance.

10. Monitor live data

Use a scan tool to monitor Pressure Sensor/Switch "B".

Observe whether the signal changes appropriately with:

• Engine load

• Transmission gear

• Vehicle speed

• Transmission temperature

11. Compare sensor data with actual hydraulic pressure

If necessary, connect a mechanical pressure gauge and compare the actual hydraulic pressure with the value reported electronically.

This can distinguish between:

A faulty electrical pressure signal

and

A genuine hydraulic pressure problem

12. Test pressure-control solenoids

Check solenoid resistance and operation according to manufacturer specifications.

13. Inspect the valve body

If the electrical circuit is correct but actual hydraulic pressure is abnormal, inspect the valve body for:

• Sticking valves

• Wear

• Contamination

• Internal leakage

14. Check for internal transmission problems

If actual hydraulic pressure remains abnormal, further investigation may be necessary for:

• Pump problems

• Internal leaks

• Worn clutches

• Damaged seals

• Hydraulic passage restrictions

15. Test the TCM

The TCM should generally be considered only after the sensor, wiring, power, reference voltage, ground and hydraulic system have been properly tested.

Can Low Transmission Fluid Cause P0845?

Yes, it can contribute to the problem.

Low fluid can cause:

• Hydraulic pressure loss

• Delayed engagement

• Slipping

• Abnormal shifting

• Pressure fluctuations

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

The electrical circuit should still be tested.

Can Dirty Transmission Fluid Cause P0845?

Yes, potentially.

Contaminated fluid can affect:

• Pressure sensors

• Pressure switches

• Solenoids

• Valve bodies

• Hydraulic passages

If the fluid contains significant debris or smells strongly burnt, further transmission inspection may be necessary.

Can Wiring Cause P0845?

Yes.

Wiring faults are an important possible cause.

A broken, corroded or shorted wire can prevent the TCM from receiving a valid pressure-sensor signal.

Because transmission wiring is exposed to heat and vibration, harness damage should be inspected carefully.

Can a Bad Pressure Sensor Cause P0845?

Yes.

The pressure sensor/switch itself can fail internally.

Possible failures include:

• Incorrect output

• No output

• Unstable output

• Internal electrical failure

• Temperature-dependent failure

A sensor that works when cold but fails when hot can be particularly difficult to diagnose.

Can P0845 Cause Limp Mode?

Yes.

If the TCM cannot obtain reliable pressure information, it may activate a protective transmission strategy.

Depending on the vehicle, this may cause:

• Fixed-gear operation

• Limited gear selection

• Higher line pressure

• Reduced performance

• Transmission warning light

Can P0845 Cause Harsh Shifting?

Yes.

The TCM uses pressure information to control hydraulic operation during gear changes.

If the information is incorrect, possible symptoms include:

• Hard shifts

• Delayed shifts

• Harsh engagement

• Shift flare

• Erratic gear changes

Can P0845 Damage the Transmission?

P0845 itself does not prove transmission damage.

However, if the transmission is operating with abnormal hydraulic pressure or incorrect clutch control, continued driving can increase wear.

The cause should therefore be diagnosed promptly.

Common Diagnostic Mistakes With P0845

Avoid these mistakes:

• Replacing the transmission immediately

• Replacing the pressure sensor without testing the circuit

• Assuming the problem is only low transmission fluid

• Ignoring wiring

• Ignoring connector corrosion

• Not checking sensor power and ground

• Not checking reference voltage

• Ignoring pressure-control solenoids

• Not monitoring live data

• Not comparing electronic readings with actual hydraulic pressure

• Replacing the TCM without testing the circuit

What Is the Correct Repair for P0845?

The correct repair depends on the actual cause.

Possible repairs include:

• Replacing the Transmission Fluid Pressure Sensor/Switch "B"

• Repairing damaged wiring

• Repairing an open circuit

• Repairing a short to ground

• Repairing a short to voltage

• Replacing damaged connector terminals

• Cleaning corroded connectors

• Repairing sensor power or ground

• Correcting reference-voltage problems

• Repairing water-damaged wiring

• Correcting 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 internal transmission leaks

• Reprogramming the TCM when required

• Replacing the TCM if confirmed faulty

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

The pressure-sensor signal should be monitored under different:

• Transmission temperatures

• Engine loads

• Gears

• Vehicle speeds

• Hydraulic-pressure conditions

Final Thoughts on P0845

P0845 – Transmission Fluid Pressure Sensor / Switch "B" Circuit indicates that the transmission control system has detected a problem in the electrical circuit associated with the "B" transmission fluid pressure sensor or switch.

This is a relatively general code. Unlike P0846, P0847, P0848 and P0849, it does not specifically tell you whether the signal is out of range, low, high or intermittent.

The fault can be caused by a bad pressure sensor, damaged wiring, loose or corroded connectors, poor ground, reference-voltage problems, or a control-module fault. In some cases, an actual hydraulic problem involving low fluid, a pressure-control solenoid, valve body or internal transmission components can also be involved.

The best diagnostic approach is to start with the sensor, connector, wiring, power, reference voltage and ground. If the electrical system is operating correctly, compare the electronic pressure reading with actual hydraulic pressure before concluding that the transmission has an internal problem.

P0845 should not be interpreted as proof that the transmission needs to be rebuilt or replaced. Proper electrical and hydraulic testing is essential before making that decision.