OBD-II trouble code
P0847: Transmission Fluid Pressure Sensor/Switch "B" Circuit Low
If this reading is honest, the damage is not spread across the transmission — it is concentrated on the one clutch that is not being held. That is why the complaint arrives as 'it slips in one gear' rather than 'the gearbox is going'.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $2,200
- DIY difficulty
- Advanced DIY
What does P0847 mean?
P0847 sets when the module reads the second transmission fluid pressure sensor's signal below the voltage any valid reading should occupy. As with any circuit-low code, that is usually electrical — but what makes this one worth reading carefully is what happens on the occasions when it is not, because a genuinely low reading on this particular channel has a narrower and more specific meaning than a low reading on the main sensor does.
The main sensor watches the supply the whole transmission works from. If that is genuinely low, everything is in trouble at once and the vehicle usually makes it obvious. The second sensor generally watches one circuit — a specific clutch or band apply passage, or the converter clutch. If the pressure there is genuinely low, the rest of the transmission is fine and one element is being applied too softly. So the damage, when there is damage, is concentrated rather than general: the clutch that is not being held properly slips against itself under load, generates heat in one place, and burns its friction material while every other element in the gearbox remains in good condition.
That is why the driver's description is so distinctive and so useful. The complaint is not that the transmission is slipping — it is that the car slips or flares in one particular gear, or that the converter clutch shudders while everything else feels normal. A transmission that is otherwise perfect but misbehaves in exactly one place is pointing straight at a single circuit, and this code names which one. Taking that complaint at face value is more informative than any generic transmission test.
On the electrical side there is a cause specific to this circuit that is worth knowing because it is both common and easily missed: recent transmission work. On most designs the second sensor is inside the pan on the valve body, and the internal harness that reaches it is disturbed every time the pan comes off — for a fluid and filter service, a solenoid replacement, or valve body work. A wire pinched under the pan lip, a connector not fully latched, a terminal disturbed while the harness was moved, or the pan bolted down over a lead sitting where it should not be will all produce a low reading. If this code appeared shortly after the transmission was serviced, that history is the strongest lead available and should be checked before a sensor is ordered.
Separating the two possibilities is straightforward in principle. If the electrical circuit tests clean and the reading is still low, put a mechanical gauge on the specified port and find out whether the pressure is actually there. If it is, the sensor is lying and gets replaced. If it is not, the sensor is right, and the search moves to the solenoid, the seals and the valve body — a different repair at a different price.
Common causes
- Short to ground in the "B" sensor signal wire
- Internal transmission harness pinched, chafed or disturbed during recent pan-off work
- Connector not fully latched or a terminal displaced after a fluid and filter service or solenoid replacement
- Failed pressure sensor "B" with its output latched low
- Corroded or fluid-contaminated terminal in the transmission case connector
- Genuinely low pressure in the monitored circuit from a leaking seal or sealing ring
- Solenoid controlling the circuit stuck partly open or not commanding full pressure
- Low fluid level, so the circuit never builds the pressure it is asked for
- Worn clutch pack or band letting the circuit fill without holding pressure
- Lost or high-resistance sensor ground, which can pull the reading down without the wiring being obviously damaged
Symptoms
- Check engine light with P0847 stored
- Slipping or flaring in one specific gear while the others feel entirely normal
- Torque converter clutch shuddering, cycling or refusing to lock up
- Engine revs rising during one particular shift without a matching increase in road speed
- Delayed or soft engagement of one gear only
- Transmission entering a fail-safe or limp mode
- Rising transmission fluid temperature, particularly on sustained motorway driving
- Code appearing within days or weeks of a transmission service or valve body repair
- No drivability complaint at all where the fault is purely electrical
Diagnostic steps
- 1.Ask whether the transmission has been serviced or opened recently. A pinched internal harness or an unlatched connector left behind by pan-off work is a leading cause of this specific code and is free to find.
- 2.Check fluid level and condition. A circuit that cannot reach commanded pressure because the fluid is low is a genuine fault and the sensor is reporting it honestly.
- 3.Take the driver's description seriously and ask which gear misbehaves. A transmission that is normal everywhere except one place is naming the circuit for you.
- 4.Note whether an "A" code is present too. "B" alone points at the "B" branch; both together point at the shared ground, reference or case connector.
- 5.Disconnect the sensor and check whether the signal line still reads low with it out of the circuit. Still low means the short is in the wiring; normal means the sensor is pulling it down.
- 6.Check the sensor ground and reference under load rather than at rest, since a lost ground produces a low reading with visually perfect wiring.
- 7.Inspect the case connector for fluid intrusion and corrosion, and treat fluid inside it as a seal problem rather than something to clean and reassemble.
- 8.If the circuit tests electrically clean, connect a mechanical gauge to the specified port and measure the real pressure during the event that pressurises the monitored circuit. This is what separates a lying sensor from a genuine leak.
- 9.If the pressure really is low, work outward to the solenoid, the sealing rings and the valve body rather than replacing the sensor that correctly reported it.
- 10.Confirm the repair with a road test that performs the relevant shift while both pressure values are graphed, rather than by clearing the code and waiting.
Repair cost
$120 – $2,200
Correcting a pinched or unlatched internal harness after a recent service is sometimes only the labour to drop the pan again, around $150 to $350 including fresh fluid and a gasket, and it is the cheapest outcome worth checking for first. External wiring or connector repair is $120 to $400. A pressure sensor is $150 to $750 fitted, with the spread driven by whether it is on the outside of the case or inside the pan on the valve body. A shift solenoid or solenoid pack is $250 to $900 fitted. If the low pressure is real and traced to seals or the valve body, $600 to $2,200 is the realistic range. Where a clutch pack has already been burnt by prolonged slipping, the repair moves into transmission overhaul territory, which is the specific reason not to leave a genuine low-pressure fault running.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission fluid pressure sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.