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DTC codes / P08xx · Transmission / P0843

P0843Transmission Fluid Pressure Sensor/Switch 'A' Circuit High

P0843 means the transmission control module (TCM) has detected an abnormally high voltage on the signal circuit of transmission fluid pressure sensor or switch 'A', which is typically the sensor that monitors line pressure inside the transmission. The computer expects a voltage within a calibrated range and sets this code when the signal stays above the high threshold, indicating an open circuit, a short to voltage, or a faulty sensor.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaTransmission

Symptoms you may notice

  • Illumination of the check engine light or transmission warning light, often with no other symptoms besides the warning light
  • Harsh or delayed shifts as the transmission defaults to a higher line pressure strategy
  • Transmission stuck in a single gear or limp-home mode with reduced performance
  • Slipping or flaring during gear changes under acceleration
  • Erratic or inconsistent shift quality that varies with temperature or load

Common causes, most likely first

  1. Open or damaged wiring in the sensor signal circuit, including a broken wire or poor connector terminal tension
  2. Transmission fluid pressure sensor 'A' internally failed and reporting a high signal
  3. Short to voltage on the signal circuit from chafed wiring contacting a power wire
  4. Corroded or fluid-contaminated electrical connector at the sensor or transmission case pass-through
  5. Failed transmission control module (TCM) driver circuit, though this is less common
  6. Low or degraded transmission fluid causing inaccurate pressure readings that push the sensor signal out of range

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P0843, then verify the sensor signal voltage with a scan tool while watching live transmission pressure data
  2. Inspect the sensor connector and harness for corrosion, chafing, or loose terminals, and perform a wiggle test while monitoring the signal
  3. Measure resistance and voltage on the sensor signal circuit with a digital multimeter (DMM) to check for an open or a short to voltage
  4. Check transmission fluid level and condition, and service the fluid if it is degraded or contaminated
  5. Replace transmission fluid pressure sensor 'A' if wiring and connectors pass inspection but the signal remains high
  6. Repair or replace the affected section of harness if a wiring fault is confirmed
  7. Clear codes and perform a relearn or adaptation reset if required, then road test to confirm the repair

What a high pressure signal really means

  • A high voltage on the signal circuit usually indicates an open circuit or a short to voltage rather than a genuinely high transmission pressure, so replacing the sensor without checking wiring often wastes money.
  • Some vehicles use a pressure switch rather than an analog sensor for this circuit, and a switch that stays open will report a high signal even when actual line pressure is normal.
  • Transmission fluid that is low or aerated can cause pressure fluctuations that momentarily push the sensor signal above the threshold, setting P0843 without any hard electrical fault.
  • A poor ground at the transmission case or TCM can elevate the sensor signal voltage and mimic a failed sensor, so ground integrity should be verified before parts replacement.
  • This code can be set by a momentary glitch during cold starts or low-voltage events, so checking for a weak battery or charging system fault is worthwhile before deeper diagnosis.

Can I keep driving with P0843?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or long trips because the transmission may be operating with incorrect line pressure and could suffer internal damage. If the transmission is in limp mode or shifting harshly, limit driving to a safe route to service.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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