What it means
The transmission fluid pressure sensor/switch B reports a circuit intermittent fault. Hydraulic pressure feedback for shift control is unreliable on this channel.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Transmission pressure sensor/switch B reports hydraulic pressure information used by the transmission controller during shift control. The controller sets P0849 when the B circuit signal cuts in and out or becomes irregular, so pressure feedback on that channel cannot be trusted consistently.
Common causes
Most likely first.
- Connector not seated
- Harness chafing intermittently
- Connector with intermittent contact under vibration
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Save freeze-frame data and check live data for sensor B while noting when the warning or shifting problem occurs.
- With the vehicle safely parked, inspect sensor B's connector for a loose fit, damaged lock, moisture, or corrosion.
- Trace the wiring to sensor B for chafing, pinching, or damage that could open the circuit as the vehicle moves.
- Monitor the B signal with a scan tool while gently moving the harness and connector to see whether it drops out.
- If the fault persists, test the B circuit and sensor against the manufacturer's wiring diagram and specifications.
Common fixes
- Reseat and secure the sensor B connector.
- Repair chafed or damaged wiring in the B circuit.
- Clean or repair connector terminals with intermittent contact.
- Replace sensor/switch B if circuit testing confirms it is faulty.
Can I keep driving?
Avoid towing, hard acceleration, and unnecessary driving if shifts are harsh, slipping, or delayed; the transmission may enter limp mode or lose reliable shift control. Stop if it slips severely or cannot engage a gear, since continued driving can worsen transmission damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.