Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent
Diagnostic Trouble Code P0849 indicates that the Powertrain Control Module (PCM) or Transmission Control Module (TCM) has detected an intermittent or erratic voltage signal from the Transmission Fluid Pressure (TFP) Sensor/Switch 'B' circuit. This means the sensor's readings are fluctuating outside of the expected parameters for the current operating conditions, suggesting a momentary loss of signal or an unstable connection.
While the vehicle may still be drivable, an intermittent transmission pressure signal can lead to harsh shifting, slipping, and potential internal transmission damage if left unaddressed. It should be diagnosed and repaired soon to prevent further transmission wear.
Common symptoms
- Illuminated Check Engine Light
- Harsh or erratic transmission shifting
- Transmission slipping between gears
- Vehicle entering 'Limp Mode' (restricted to a single gear)
- Decreased fuel efficiency
Likely causes
- Low or dirty transmission fluid
- Faulty Transmission Fluid Pressure (TFP) Sensor/Switch 'B'
- Loose, corroded, or damaged wiring and connectors in the TFP sensor circuit
- Mechanical internal transmission failure affecting fluid pressure
- Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Fluid that is significantly low, burnt, or heavily contaminated with debris indicates a fluid issue or internal transmission wear that could cause pressure fluctuations.
Inspect the wiring harness and connectors for the TFP Sensor 'B'.
Look for: Look for loose connections, frayed wires, corrosion, or signs of melting that could cause an intermittent electrical connection.
Monitor the TFP Sensor 'B' data PID using a scan tool while driving or wiggling the harness.
Look for: Observe the pressure readings for erratic spikes or dropouts that do not correspond to actual transmission operation, which confirms an intermittent sensor or wiring fault.
Test the TFP Sensor 'B' circuit for continuity and resistance using a digital multimeter.
Look for: Resistance values that are out of the manufacturer's specified range or an open/short circuit in the wiring between the sensor and the TCM.
Perform a manual transmission fluid pressure test using a mechanical gauge.
Look for: Compare the mechanical gauge readings to the scan tool data; if the mechanical pressure is steady but the sensor data is erratic, the sensor or circuit is faulty.
This page explains the generic P0849 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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