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DTC codes / P08xx · Transmission / P0849

P0849Transmission Fluid Pressure Sensor/Switch 'B' Circuit Intermittent

P0849 means the transmission control module (TCM) has detected an intermittent electrical fault in the circuit for transmission fluid pressure sensor or switch 'B'. The TCM sees the signal from that sensor drop out or jump erratically for brief moments, rather than reading a steady pressure value, which points to a wiring or connection problem more often than a failed sensor.

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 symptoms besides the warning light
  • Harsh or delayed gear shifts, especially when the transmission is cold or under light throttle
  • The transmission may slip briefly or hesitate before engaging a gear
  • Intermittent limp mode or reduced power, sometimes clearing after a restart
  • Rough or erratic shift quality that comes and goes with bumps or temperature changes

Common causes, most likely first

  1. Chafed, pinched, or corroded wiring in the sensor 'B' circuit, often near a connector or where the harness bends
  2. Loose or contaminated electrical connector at the pressure sensor or switch 'B'
  3. Internal fault in the transmission fluid pressure sensor or switch 'B' itself
  4. Poor ground or reference voltage supply shared with other transmission sensors
  5. Transmission fluid contamination or low fluid level affecting pressure signal stability
  6. Failing transmission control module (TCM) or a software calibration issue, though this is less common

How to diagnose and fix it

  1. Scan all modules for stored and pending codes, and record freeze-frame data for P0849 to see when the fault occurs
  2. Inspect the sensor 'B' connector and the harness for corrosion, loose pins, or chafing, and wiggle-test the wiring while watching live pressure data
  3. Check transmission fluid level and condition according to the manufacturer's procedure, and top up or service if needed
  4. Measure resistance and voltage on the sensor 'B' circuit at the sensor and at the TCM, comparing to factory specifications
  5. Back-probe the signal wire and monitor live data on a road test to catch the intermittent dropout
  6. Replace the transmission fluid pressure sensor or switch 'B' if circuit tests pass but the signal still drops out
  7. If the fault persists after sensor replacement, check TCM grounds and power feeds, and reflash or replace the TCM only after all circuit tests are conclusive

Intermittent faults hide in the harness

  • Most P0849 cases are caused by a wiring or connector problem, not by a failed pressure sensor, so replacing the sensor first often wastes money and does not fix the code.
  • The fault is intermittent by definition, which means a static resistance check can pass while the circuit still fails on a bumpy road or when the harness flexes, so a wiggle test with live data is essential.
  • A low or contaminated transmission fluid level can make the pressure signal unstable and set this code, so fluid condition should be checked before any electrical parts are replaced.
  • P0849 can be set by a poor ground shared with other transmission sensors, so a fault in a neighbouring circuit can masquerade as a sensor 'B' problem.
  • Some vehicles store P0849 only as a pending code with no drivability complaint, and clearing it without checking freeze-frame data can hide a connector that is about to fail completely.

Can I keep driving with P0849?

You can usually drive short distances to a shop, but avoid heavy throttle, towing, and long highway trips until the fault is diagnosed, because an intermittent pressure signal can cause harsh shifts or a sudden limp mode that reduces power.

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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