P0847
Transmission Fluid Pressure Sensor B Low
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Code P0847 is set when the Transmission Control Module (TCM) detects that the voltage signal from Transmission Fluid Pressure Sensor B is lower than the expected operating range. This sensor monitors hydraulic pressure within a specific circuit of the automatic transmission and relays that data to the TCM, which uses it to precisely control shift timing and quality. A low signal can mean the sensor is malfunctioning, the actual fluid pressure is too low, or there is an electrical fault in the circuit between the sensor and the TCM.
The most important first step is to check the transmission fluid level and condition. Low or dirty fluid is a common and easy-to-miss cause of abnormally low pressure readings. If the fluid level is correct, the next step is to inspect the wiring and connector at the sensor for corrosion, damage, or loose pins. If the wiring checks out, the sensor itself is a strong suspect and is generally an affordable part to replace. A mechanic can use a scan tool to monitor live sensor voltage data to confirm whether the sensor is truly out of range or if the issue is in the wiring.
Ignoring P0847 is not recommended because consistently low transmission fluid pressure can cause excessive wear on transmission clutch packs and bands, leading to costly internal damage over time. Many vehicles will also enter a protective limp mode, restricting driving to a single gear to prevent further damage. Addressing the root cause promptly — whether it is a simple fluid top-off, a sensor swap, or an electrical repair — can save vehicle owners from a much more expensive transmission rebuild or replacement down the road.
Severity
Symptoms
- •Transmission slipping or hesitating during gear shifts
- •Harsh or delayed shifting between gears
- •Check engine light illuminated
- •Transmission may enter limp mode (stuck in one gear)
- •Reduced fuel economy
- •Transmission overheating warning
Likely Causes
The sensor itself has failed internally and is sending a voltage signal below the expected range to the TCM. This is one of the most common causes and typically requires sensor replacement.
Insufficient fluid in the transmission reduces hydraulic pressure, causing the sensor to read abnormally low values. A fluid check should always be the first diagnostic step.
Broken wires, corroded terminals, or a damaged connector at the pressure sensor can cause the signal voltage to drop below threshold. This is particularly common in high-mileage vehicles or those exposed to moisture.
A failing transmission pump, clogged hydraulic passages, or a stuck pressure control solenoid can cause genuinely low fluid pressure in the B circuit, triggering this code.
In rare cases, the TCM itself may misread the sensor signal or have an internal fault. This is typically only considered after all other causes have been ruled out.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Check and top off transmission fluid to the correct level using the proper fluid type
- Inspect and clean or replace the wiring harness connector at the transmission pressure sensor
- Replace the Transmission Fluid Pressure Sensor B
- Inspect and replace the transmission pressure control solenoid if internal hydraulic pressure is confirmed low
- Perform a transmission fluid flush and filter replacement if fluid is contaminated or degraded