P0849 OBD-II Trouble Code: Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent

diagnostic check engine light

P0849 code definition

The P0849 code identifies a problem with fluid levels within the transmission. The powertrain control module controls a number of internal components within an automatic transmission, and the same control module is capable of detecting this fault. If the pressure at the pump within the transmission is not at the level required by factory specifications, then it will store a code via the powertrain control module.

What the P0849 code means

P0849 is the OBD-II generic code for a fault involving the pressure of fluid within an automatic transmission. The fluid inside of the transmission is responsible for tasks such as shifting into and out of gear, along with controlling the lockup of the torque converter. Pressure is maintained by a pump that regulates the fluid pressure. All of these components are controlled by the powertrain control module. Throttle position, engine speed, vehicle speed, and engine load are used to calculate the proper gear for the transmission to operate in. Without proper fluid pressure, the transmission may not be able to do that job effectively.

What causes the P0849 code?

The P0849 code has a number of common causes:

  • Blockage within hydraulic lines inside of transmission
  • Faulty solenoid within transmission
  • Faulty sensors within the transmission
  • Fault or electrical short in the powertrain control module
  • Low transmission fluid
  • Dirty or contaminated transmission fluid
  • Mechanical failure of transmission internal parts

What are the symptoms of the P0849 code?

Since the P0849 code deals with a problem with fluid levels in the transmission, there are a number of symptoms that accompany its appearance that are noticeable to the driver. If the vehicle has a warning light on the dashboard that is specifically intended to warn the driver of transmission issues, then this light should illuminate as soon as the required pump pressure within the transmission is not detected. After a few more instances of this fault, the Check Engine light should illuminate.

Physical symptoms such as rough shifting and slipping within the transmission will also accompany this fault. In severe cases, the transmission temperature may increase dangerously and fuel economy may suffer.

How does a mechanic diagnose the P0849 code?

The P0849 code will be diagnosed by a certified technician using an OBD-II scanner that is capable of retrieving the factory readings from the vehicle. The mechanic will view the fault code and see when it originally occurred. Then, the code will be reset and the vehicle will be taken for a test drive. If the code appears once again during this test drive, then the technician can confirm that the problem is not intermittent.

To test the electrical components around the circuits controlling the automatic transmission, the technician will then use a voltmeter to test various points around the circuit to confirm that proper amounts of power are flowing through the circuits. After all of that has been completed, the technician will check the transmission fluid itself. Low, contaminated, or old transmission fluid can cause the P0849 code to appear.

Common mistakes when diagnosing the P0849 code

Often the P0849 code is interpreted as a serious problem within the automatic transmission. While this may be true, it is just as likely that the problem is either electrical or due entirely to old, low, or dirty fluid in the transmission.

How serious is the P0849 code?

The P0849 code can range from moderately serious to very serious, depending on the root of the problem. If this code appears, it should certainly be diagnosed by a qualified technician promptly. In serious cases, there are parts within the transmission that need to be replaced before the vehicle can run properly once again. In moderate cases, the technician will simply have to add more fluid to the transmission. Having this problem addressed as soon as it appears it the best way to avoid costly major repairs down the road.

What repairs can fix the P0849 code?

The most common repairs to address the P0849 code are as follows:

  • Repair or replace wiring harness around transmission
  • Repair or replace powertrain control module
  • Repair or replace connections among wiring surrounding transmission or powertrain control module
  • Replace transmission fluid
  • Rebuild or replace automatic transmission

Additional comments for consideration regarding the P0849 code

The P0849 code can appear for a number of reasons relating to electrical issues or issues with the transmission fluid itself. Have the problem thoroughly diagnosed by a qualified technician prior to replacing any components in or around the transmission.

Need help with a P0849 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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