P0849
Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 8
- AreaTransmission
- 49
- FaultThe specific fault in that area
What it means
P0849 is the generic code for an intermittent fault in the transmission fluid pressure sensor or switch 'B' circuit. The transmission control unit saw the signal from that sensor or switch drop out, spike or jump to an implausible value for short periods, rather than stay faulty all the time. Which hydraulic circuit 'B' refers to depends on the make, model and transmission, so check the manufacturer's information for your vehicle.
Symptoms
- Gearbox warning light or malfunction indicator lamp on
- Intermittent harsh, delayed or flared gear changes
- Transmission dropping into limp mode or a fixed gear now and then
- Occasional loss of lock-up or erratic shift timing
- Problem appears and disappears, often with bumps, heat or vibration
Possible causes
- Intermittent connection at the pressure sensor or switch connector, such as corrosion, bent pins or a loose fitlikely
- Damaged, chafed or fluid-soaked wiring between the sensor and the transmission control unitlikely
- Failing pressure sensor or switch giving an unstable signalpossible
- Low, degraded or contaminated transmission fluid causing unstable pressure readingspossible
- Internal transmission harness or valve body connector fault (design depends on the transmission)possible
- Hydraulic fault such as a sticking valve or worn internal seals causing real pressure fluctuationsrare
- Fault within the transmission control unit itself, or an outdated calibration where the manufacturer has issued a known updaterare
How to diagnose
- Read all stored codes and freeze frame data, noting when the fault occurred (speed, gear, temperature) and whether other transmission codes are present
- Check the transmission fluid level and condition using the manufacturer's procedure, as some units are sealed and need specific temperature conditions
- Inspect the sensor or switch connector and the wiring for damage, chafing, corrosion, fluid ingress and poor pin tension
- With the engine running, wiggle the harness and connector while watching live pressure sensor or switch data for dropouts
- Check the sensor's reference voltage, earth and signal wiring for continuity, short circuits and resistance against the manufacturer's specification
- Check for technical service bulletins or calibration updates for your model and transmission
- Test the sensor or switch against the manufacturer's data, and measure actual line pressure if the signal appears genuine
- Clear the code and road test to confirm whether the fault returns
How to fix
- Clean, repair or replace the affected connector and terminals
- Repair or replace damaged wiring or the internal transmission harness
- Replace the pressure sensor or switch if it fails testing
- Correct the fluid level, or change the fluid and filter if it is degraded, using the specified type
- Repair the valve body or internal hydraulic components if line pressure is confirmed faulty
- Apply a manufacturer-issued control unit calibration update only if a bulletin identifies it as a cause
Can I keep driving?
The car can usually be driven for a short time, but the gearbox may enter limp mode or shift harshly without warning. Have it checked soon, as continued driving with real pressure problems could damage the transmission.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.