P0847
Transmission Fluid Pressure Sensor/Switch B Circuit LowP0847 is a generic OBD-II powertrain diagnostic trouble code: Transmission Fluid Pressure Sensor/Switch B Circuit Low. It is logged by the engine control unit when the trans monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0847 means
P0847 — Transmission Fluid Pressure Sensor/Switch "B" Circuit Low — is set when the PCM or TCM measures a voltage on the sensor B signal wire that falls below the manufacturer's minimum threshold. This electrical condition is consistent with an open circuit in the signal wire, a short to ground, or a sensor whose internal resistance has fallen, pulling the signal line down toward 0 V. The control module cannot determine actual line pressure and must revert to a default operating strategy.
With the sensor reporting a falsely low (or absent) pressure signal, the TCM may command maximum line pressure to compensate, resulting in harsh and abrupt gear changes. Alternatively, the module may interpret the low signal as critically low hydraulic pressure and engage fail-safe mode, locking the transmission in a single gear. In either case, continued operation risks overloading clutch packs and accelerating internal wear.
On Honda and some Chrysler platforms the P0847 definition is strictly electrical — the manufacturer explicitly states the code cannot be caused by a mechanical transmission problem on those vehicles. On GM and Ford applications, a severely low actual line pressure (pump failure, major internal leak) can also produce a legitimately low sensor output alongside the code.
Common causes
Most-frequently reported root causes when P0847 is logged.
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1
Open circuit in the transmission fluid pressure sensor B signal wire (broken wire or corroded terminal).
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2
Short to ground on the sensor signal circuit, pulling the voltage below the low threshold.
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3
Failed transmission fluid pressure sensor B with internally shorted or open sensing element.
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4
Damaged wiring harness connector with spread, corroded, or missing terminals at the sensor plug.
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5
Low transmission fluid level producing actual line pressure below the sensor's measurable range.
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6
Faulty pressure control solenoid causing actual hydraulic pressure to drop abnormally low.
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7
Transmission pump failure or major internal hydraulic leak reducing actual line pressure.
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8
PCM/TCM internal fault incorrectly reading the sensor signal circuit voltage.
Symptoms drivers notice
How to diagnose P0847
A typical diagnostic flow when this code is present.
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1
Check transmission fluid level and condition first; extremely low fluid can cause genuinely low line pressure and a valid circuit-low reading.
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2
With the ignition on and sensor connector unplugged, verify the reference voltage supply (typically 5 V) at the harness connector using a DMM.
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3
Check for ground integrity at the sensor ground pin; a high-resistance ground can mimic a circuit-low condition.
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4
With the connector still unplugged, measure resistance of the signal wire back to the TCM — infinite resistance indicates an open; near-zero resistance to ground indicates a short to ground.
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5
Reconnect the sensor and measure the live signal voltage; a reading near 0 V with the engine running confirms the circuit-low condition.
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6
Substitute a known-good transmission fluid pressure sensor B and re-evaluate — if voltage returns to normal range, the sensor has failed internally.
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7
If wiring and sensor check out, perform a mechanical line pressure test with a gauge to rule out a hydraulic fault before inspecting the TCM.
Related powertrain codes
- P0218 — Transmission Over Temperature Condition
- P0700 — Transmission Control System Malfunction
- P0701 — Transmission Control System Range/Performance
- P0702 — Transmission Control System Electrical
- P0703 — Torque Converter/Brake Switch B Circuit Malfunction
- P0705 — Transmission Range Sensor A Circuit malfunction (PRNDL Input)
Frequently asked questions
Can a short to ground in the wiring set P0847?
Yes. A signal wire shorted to the chassis or transmission case ground will hold the sensor input at or near 0 V regardless of actual line pressure, triggering the circuit-low threshold and storing P0847.
Why does my transmission shift so harshly with P0847 active?
When the sensor reports zero or sub-threshold pressure, the TCM often commands maximum line pressure as a protective default. This causes very firm, abrupt clutch engagement — the transmission is trying to prevent slipping under what it perceives as critically low hydraulic pressure.
Is P0847 purely electrical on all vehicles?
Not on all platforms. Honda explicitly states P0847 is caused only by an electrical circuit fault on their vehicles. On GM, Ford, and some Chrysler applications, actual hydraulic pressure loss from a failing pump or major internal leak can produce a genuinely low sensor signal that accompanies the code.
How do I tell if it is the sensor or the wiring causing P0847?
Disconnect the sensor connector and check resistance of the signal wire to ground (short to ground) or continuity back to the TCM (open circuit). If the wiring passes both tests, substitute the sensor. If voltage returns to normal range with a new sensor installed, the original sensor failed internally.
Disabling P0847 in software
RaceTune can permanently disable P0847 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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