Fault code P0B04 – auxiliary transmission fluid pump motor phase “V” current low

Fault code P0B04 is called “Auxiliary Transmission Fluid Pump Motor Phase “V” Current Low” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0B04

This diagnostic trouble code (DTC) is a generic transmission code. Error P0B04 is considered a general code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly depending on the model.

Fault code P0B04 – auxiliary transmission fluid pump motor phase

Modern vehicles have an automatic transmission control module called a transmission control module (TCM). The Powertrain Control Module (PCM) communicates with the TCM. To monitor the automatic transmission for malfunctions.

If your vehicle retains code P0B04, it means that the powertrain control module (PCM) has received a data input signal. Which indicates that a problem has been detected that requires the malfunction indicator lamp (MIL) to turn on.

The transmission controller activates this error when it detects a malfunction in the auxiliary transmission pump motor. In this case, the problem is related to low current in the “V” phase of the auxiliary transmission pump motor.

Some models may require up to 8 driving cycles with the problem before the malfunction indicator illuminates.

Symptoms of vehicle malfunctions

The main signal that an error P0B04 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. Gear mode switch does not work.
  2. Unstable or abrupt gear shifting.
  3. Takes a long time to shift gears.
  4. Low power at high load.
  5. Transmission may be put into emergency mode.

A stored code P0B04 indicates that a potentially serious fault has been detected. This may cause the transmission and its components to fail.

Factors that can cause this error code

The error code P0B04 can mean that one or more of the following problems have occurred:

  • Defective auxiliary transmission fluid pump.
  • Corrosion of connectors or contacts causing high resistance.
  • Problem with wiring, e.g. breakage.
  • Defective drive motor.
  • Defective transmission fluid pump control relay.
  • Blown fuse or fusible link.
  • Sometimes faulty PCM or TCM module is the cause.

How to fix or reset OBD-2 code P0B04

Some suggested steps for troubleshooting and fix the error code P0B04:

  1. Connect an OBD-II scanner to the vehicle’s diagnostic connector and read all stored data and error codes.
  2. Clear the error codes, perform a test drive, and check if the P0B04 fault appears again.
  3. Visually inspect the electrical wires and connectors.
  4. If necessary, repair or replace any shorted, broken, damaged, or corroded components.
  5. Check the fuse or fusible link.
  6. Inspect the auxiliary transmission fluid pump.
  7. Test the pump control relay.

Diagnose and repair of problems

The first thing to do when starting to diagnose a P0B04 fault is to check the electrical part for damage. In modern vehicles, electrical and electronic signals require good cabling for proper data transmission. Any faults, such as breaks, disconnected connectors, or corrosion, can interrupt signals, resulting in transmission control failures.

Check the battery pack, as some PCM and TCM modules are sensitive to low voltage. If the battery is low, the system may detect this as a failure. Make sure the battery is delivering at least 12 volts and that the alternator is running properly, at least 13 volts at idle.

If the car does not detect a failure caused by faulty wiring, the problem is most likely with the auxiliary transmission fluid pump itself. Its replacement should solve the problem with the P0B04 error. But don’t overlook fuses either, as a blown fuse is a common problem.

As with any transmission malfunction, always make sure the fluid levels and quality are at the proper level, and keep them in good condition.

On which vehicles does this problem occur most frequently

Fault code P0B04 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Mercedes-Benz
  • Toyota

Fault code P0B04 can sometimes be found with other errors. The most common are the following: P0B00, P0B01, P0B02, P0B03, P0B05, P0B06, P0B07, P0B08.


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