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Home / DTC / P0843 — Trans Fluid Press Sensor/Switch A Circuit High

P0843 — Trans Fluid Press Sensor/Switch A Circuit High

Detailed page for trouble code P0843.

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Code

P0843

MERCEDES-BENZ P — Powertrain

Trans Fluid Press Sensor/Switch A Circuit High

Brand: MERCEDES-BENZ
Type: P — Powertrain
AI status
Completed
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Page language: EN

Causes

  • Short to battery/ignition 12V or to the 5V reference circuit
  • Open or high-resistance wiring between sensor and TCM
  • Corroded, pushed-out, or damaged sensor connector pins
  • Failed transmission fluid pressure sensor/switch (internal short or internal electronics fault)
  • Poor ground at the sensor or TCM
  • Faulty TCM or internal TCM input driver (less common)

Symptoms

  • Malfunction Indicator Lamp (MIL) or transmission warning light illuminated
  • Transmission may go into limp mode / reduced function
  • Erratic shifting, harsh or delayed shifts, or failure to shift
  • Transmission may default to a single gear
  • Possible stored transmission-related fault codes

What to check

  • Read and record DTC(s) and freeze-frame / readiness data with an OBD-II scanner
  • Check battery voltage and charging system before electrical testing
  • Visually inspect the transmission fluid pressure sensor connector and wiring for corrosion, damaged insulation, pin damage, or pushed-out pins
  • Backprobe signal, reference and ground wires at the sensor with a multimeter or scan tool while key ON and engine running
  • Compare sensor voltage to known-good ranges and watch live-data while changing engine load/gear
  • Wiggle the wiring harness along its routing to reproduce the fault

Signal parameters

  • Sensor type: typically 3-wire pressure sensor using +5V reference, ground, and signal to TCM
  • Expected signal voltage (typical): ~0.5–4.5 V proportional to pressure (varies by model)
  • Circuit-high condition: signal stuck near supply voltage (e.g., >4.8 V) or above the expected maximum
  • Reference voltage at sensor: approx. 5.0 V (key ON)
  • Ground: near 0 V; significant voltage on ground indicates poor ground/connection

Diagnostic algorithm

  1. Verify DTC P0843 is current. Capture freeze-frame and live-data for the pressure sensor signal, reference voltage, battery voltage, and engine speed.
  2. Visually inspect the sensor connector and harness for corrosion, bent pins, damage, or fluid intrusion. Repair connectors if damaged.
  3. With key ON (engine OFF) backprobe the sensor: verify reference ~5.0 V, signal voltage, and good ground. Note signal value (circuit-high will show near supply voltage).
  4. Wiggle the harness while watching live-data to check for intermittent opens/shorts.
  5. If reference is present and signal is high, inspect for a short to 12V or to the 5V supply on the signal wire: isolate by disconnecting the sensor and measuring voltage at the harness plug. If signal remains high with sensor disconnected, likely short in wiring or TCM.
  6. Check continuity between sensor ground and chassis ground; repair any poor ground connections.
  7. Measure resistance/continuity from the sensor signal pin to the TCM signal pin. If open or abnormal, repair wiring harness.
  8. If wiring and connectors are good and signal only abnormal with the sensor connected, replace the pressure sensor/switch with a known-good unit and retest.
  9. After repair, clear codes and perform a road test while monitoring live-data to confirm the signal returns to expected range and no new codes set.
  10. If fault persists despite good wiring and a known-good sensor, suspect TCM input or internal electronics and consult manufacturer procedures for TCM testing or replacement.

Likely causes

  • Corroded/damaged sensor connector or wiring (most common)
  • Short to 5V or battery positive on the signal circuit
  • Faulty pressure sensor/switch

Fault status

⚠️ Status
Transmission fluid pressure sensor/switch A circuit high — signal above expected range (possible short to voltage, open wiring, or sensor fault).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

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Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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