What it means
The transmission fluid pressure sensor/switch B reports a circuit low 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 or switch B sends a voltage signal to the control module for its monitored hydraulic pressure channel. P0847 sets when that circuit voltage falls below the calibrated threshold, which can result from a signal short to ground, an internally shorted sensor, or a poor connector connection.
Common causes
Most likely first.
- Signal wire shorted to ground
- Internal sensor short
- Connector pins corroded - high contact resistance
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
- Record freeze-frame data and check sensor B live voltage or pressure data to confirm whether the signal is low when the fault is active.
- Inspect sensor B’s connector and harness for corrosion, fluid intrusion, damaged insulation, or a signal wire touching ground.
- Using the wiring diagram, verify the B sensor reference supply and ground, then check the signal wire for a short to ground.
- Test the B circuit and connector terminals against the manufacturer’s specification under the prescribed conditions.
- If the wiring and supply test correctly, check sensor B for an internal short using the vehicle’s diagnostic procedure.
Common fixes
- Clean and secure loose or corroded sensor B connector terminals where appropriate.
- Repair a signal wire short to ground or other damaged B-circuit wiring.
- Replace sensor B if its output remains below specification with a correctly functioning circuit.
- Repair the source of fluid intrusion if it has contaminated the connector.
Can I keep driving?
Because a low B-circuit signal can cause abnormal shifting or limp mode, avoid towing, hard acceleration, and unnecessary trips. Stop driving if the transmission slips, loses drive, or becomes stuck in a gear, since continued operation can cause 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.