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Home / DTC / P0842 — Trans Fluid Press Sensor/Switch A Circuit Low

P0842 — Trans Fluid Press Sensor/Switch A Circuit Low

Detailed page for trouble code P0842.

34,405codes
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22,491specific
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Code

P0842

MERCEDES-BENZ P — Powertrain

Trans Fluid Press Sensor/Switch A Circuit Low

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

Causes

  • Short to ground in the sensor signal wire
  • Open or high‑resistance wiring/connectors between sensor and control module
  • Corroded or poorly seated sensor connector
  • Failed transmission fluid pressure sensor or switch
  • Low or no transmission hydraulic pressure (internal fault, low fluid level)
  • Faulty TCM/ECM or poor module ground

Symptoms

  • Check Engine Light (MIL) or transmission warning light illuminated
  • Transmission may enter limp/backup mode
  • Harsh, delayed, or erratic shifting
  • Erratic line pressure behavior (slip, harsh shifts)
  • Stored transmission-related trouble codes

What to check

  • Read freeze frame and full code list with a capable scan tool; record live pressure sensor/switch A data
  • Visually inspect wiring and connector at the pressure sensor for corrosion, damage, pin push‑out, and proper sealing
  • Check and confirm correct transmission fluid level and condition per manufacturer procedure
  • Backprobe sensor connector with key ON (engine OFF) to check reference and signal voltages
  • Check for short to ground on the signal circuit (ohmmeter or wiring map)
  • Wiggle test wiring while observing live data to locate intermittent faults

Signal parameters

  • Typical reference voltage: 5 V (verify exact value from factory service data)
  • Typical signal range: about 0–5 V proportional to pressure (vehicle-specific)
  • Low/failed condition: signal ≈ 0–0.5 V or below expected range
  • High condition (different code): signal ≈ >4.5 V (not P0842)
  • Resistance to ground on signal circuit should not indicate a direct short (low ohms) — consult wiring diagram for expected values

Diagnostic algorithm

  1. Verify the code with a scan tool, record freeze frame and live data for pressure sensor A, then clear codes and attempt to reproduce.
  2. Visually inspect the sensor and connector for damage, corrosion, fluid intrusion, or loose pins. Repair if found.
  3. With the ignition ON (engine OFF), backprobe the connector: confirm reference voltage (often 5 V) and an expected signal voltage. If reference is missing, suspect power/ECU/TCM circuit.
  4. Check the signal circuit for a short to ground: disconnect the sensor and measure continuity between signal pin and chassis ground. Low resistance indicates a short.
  5. If wiring checks OK, monitor signal voltage with engine running or during a pressure event. If signal stays near 0 V while pressure should be present, suspect defective sensor.
  6. Verify hydraulic pressure with a mechanical transmission pressure gauge and compare to sensor readings to determine if the issue is electrical or mechanical.
  7. Repair wiring/connectors as needed. Replace the pressure sensor/switch if wiring and module outputs are good.
  8. After repairs, clear codes and road test to confirm proper shift behavior and that the code does not return. If code persists with verified wiring and sensor, consider module diagnosis or replacement per manufacturer procedure.

Likely causes

  • Damaged/chafed signal wire shorted to chassis ground
  • Corroded connector pins at the pressure sensor
  • Failed pressure sensor/switch (common)
  • Low fluid level or pump failure causing true low pressure
  • Intermittent wiring or connector fault

Fault status

⚠️ Status
Stored DTC P0842 — Transmission Fluid Pressure Sensor/Switch A Circuit Low: low or no signal detected from transmission pressure sensor A. Possible short to ground, sensor failure, poor connector, low fluid pressure, or control module issue.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-3 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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