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DTC codes / P08xx · Transmission / P0847

P0847Transmission Fluid Pressure Sensor/Switch 'B' Circuit Low

P0847 indicates that the transmission control module (TCM) has detected an abnormally low voltage on the signal circuit for transmission fluid pressure sensor or switch 'B'. This sensor monitors hydraulic pressure inside the transmission, and a low signal can prevent the TCM from regulating line pressure correctly, potentially affecting shift quality and transmission durability.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other noticeable symptoms
  • Harsh or delayed gear shifts, especially when the transmission is cold
  • The transmission may enter limp mode, limiting vehicle speed and gear selection
  • Slipping or flaring during gear changes under acceleration
  • Erratic or inconsistent shift patterns that vary with engine load
  • In some cases, the vehicle may fail to engage certain gears or hesitate before moving

Common causes, most likely first

  1. Faulty transmission fluid pressure sensor or switch 'B' itself, causing a low signal output
  2. Damaged, chafed, or corroded wiring in the sensor's signal circuit, leading to a short to ground or high resistance
  3. Loose or contaminated electrical connector at the sensor or TCM
  4. Low transmission fluid level or degraded fluid affecting actual hydraulic pressure and sensor readings
  5. Internal transmission hydraulic issues, such as a stuck valve or worn pump, causing genuinely low pressure
  6. A malfunctioning TCM or a software calibration issue that misinterprets the sensor signal
  7. Poor ground connection for the sensor or TCM, causing reference voltage instability

How to diagnose and fix it

  1. Check the transmission fluid level and condition; top off or replace fluid if it is low or burnt, as this is the cheapest and simplest first step
  2. Inspect the sensor and its connector for corrosion, loose pins, or damage, and clean or repair as needed
  3. Perform a visual inspection of the wiring harness between the sensor and TCM, looking for chafing, pinches, or signs of overheating
  4. Use a digital multimeter to measure the sensor's signal voltage and compare it to the manufacturer's specification; a low reading confirms a circuit or sensor fault
  5. Test the sensor's resistance or output with a scan tool capable of reading transmission data, and compare live values to expected ranges
  6. If the sensor tests faulty, replace it and clear the code; if the sensor is good, check for shorts to ground or power in the signal circuit
  7. If all external checks pass, diagnose internal transmission hydraulic pressure with a gauge and inspect valve body components, which may require professional service

What P0847 often hides behind the sensor

  • A low signal from sensor 'B' is frequently caused by a chafed wire grounding against the transmission case rather than a failed sensor, so always inspect the harness before replacing parts.
  • Low transmission fluid level or a clogged filter can create genuinely low hydraulic pressure that mimics a sensor fault, so fluid condition is a critical first check.
  • Some vehicles use sensor 'B' for specific pressure ranges only, and a failure may not trigger noticeable symptoms until the transmission is under heavy load or during cold starts.
  • A faulty ground connection shared with other sensors can pull the reference voltage down and set P0847 even when the sensor and its wiring are intact.
  • Replacing the sensor without clearing adaptive learning values can leave the TCM with incorrect shift pressure adaptations, causing continued poor shift quality until relearn is performed.

Can I keep driving with P0847?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or sustained highway speeds because low pressure can cause clutch slip and internal damage. If the transmission enters limp mode or shifts harshly, stop driving and seek professional diagnosis to prevent further wear.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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