P0843
Transmission Fluid Pressure Sensor/Switch A Circuit HighP0843 is a generic OBD-II powertrain diagnostic trouble code: Transmission Fluid Pressure Sensor/Switch A Circuit High. 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 P0843 means
P0843 is triggered when the PCM or TCM detects a voltage signal from the transmission fluid pressure sensor or switch 'A' that is higher than the expected operating range. The transmission fluid pressure sensor (TFPS) is a critical monitoring device that measures hydraulic pressure inside the transmission and reports it to the control module. The module uses this reading to calibrate shift timing, torque converter lockup, and line pressure adjustments. When the signal voltage climbs above the calibrated threshold and stays there, P0843 is logged.
An abnormally high voltage signal typically indicates one of three scenarios: the sensor itself has failed and is outputting an incorrect voltage; there is a short to voltage somewhere in the signal or reference circuit wiring; or actual transmission fluid pressure is genuinely elevated beyond normal parameters due to a stuck pressure regulator valve or blocked return circuit. Distinguishing between an electrical fault and a real over-pressure condition is a key step in diagnosis.
Because the TCM cannot trust the pressure data it receives, it may default to a conservative or fixed line-pressure strategy. This often manifests as harsh, firm shifts — the transmission behaves as though maximum pressure is always applied — or it may trigger limp mode to prevent further damage. In some vehicles the TCM logs additional companion codes pointing to specific solenoids or circuits that should be investigated alongside P0843.
Transmission fluid condition is always worth checking first: heavily degraded fluid can cause erratic sensor readings and real pressure anomalies simultaneously. However, on many vehicles a failed pressure switch is the most frequent root cause and represents a relatively straightforward repair before assuming deeper hydraulic issues.
Common causes
Most-frequently reported root causes when P0843 is logged.
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1
Failed transmission fluid pressure sensor/switch 'A'
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2
Short to voltage in the sensor signal or reference circuit wiring
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3
Damaged, corroded, or loose connector at the pressure sensor
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4
Genuine over-pressure condition due to stuck pressure regulator valve
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5
Clogged transmission fluid passages causing pressure build-up
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6
Degraded or contaminated transmission fluid
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7
Faulty PCM/TCM (rare, after ruling out sensor and wiring)
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8
Incorrect fluid type causing abnormal pressure characteristics
Symptoms drivers notice
How to diagnose P0843
A typical diagnostic flow when this code is present.
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1
Retrieve all stored DTCs; note companion transmission codes that may narrow the fault to a specific circuit
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2
Inspect transmission fluid level and condition — check for discolouration, burnt smell, or metallic particles
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3
Locate the transmission fluid pressure sensor/switch 'A' and perform a visual inspection of the connector and wiring for corrosion, fraying, or damaged insulation
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4
With a multimeter, measure the signal circuit voltage at the sensor connector with key-on engine-off — a reading near battery voltage (12 V) with no sensor connected indicates a wiring short to power
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5
Test the sensor resistance or voltage output across its specified pressure range and compare to manufacturer specifications
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6
Conduct a hydraulic line pressure test to confirm whether actual transmission pressure is elevated or within spec
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7
If wiring and sensor test good, suspect a stuck pressure regulator valve in the valve body
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8
Replace PCM/TCM only after all other causes have been definitively ruled out
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
Is the transmission fluid pressure switch the same as a sensor?
They serve the same purpose but differ in output. A switch is binary — open or closed at a threshold pressure. A sensor provides a continuous variable voltage proportional to pressure.
Will P0843 damage my transmission if I keep driving?
Potentially yes. The TCM may default to maximum line pressure to compensate for the faulty signal, which causes accelerated wear on clutch packs and bands.
How do I know if it is a sensor fault or an actual pressure problem?
Perform a manual hydraulic line pressure test using a mechanical gauge. If measured pressure matches specification, the sensor or its circuit is at fault.
Can low transmission fluid cause P0843?
Low fluid is more commonly associated with low-pressure codes (P0841, P0842). P0843 specifically indicates a high signal, but severely degraded or aerated fluid can create erratic pressure spikes.
Disabling P0843 in software
RaceTune can permanently disable P0843 — 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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