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Home / DTC / P0849 — Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent

P0849 — Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent

Detailed page for trouble code P0849.

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11,643generic
22,491specific
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Code

P0849

Generic P — Powertrain

Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent

Brand: Generic
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Corroded, loose, damaged or disconnected sensor connector or wiring harness (intermittent contact)
  • Damaged insulation causing intermittent short to voltage or ground
  • Faulty transmission fluid pressure sensor/switch B (internal intermittent)
  • Intermittent poor ground or reference voltage to the sensor
  • Water intrusion or corrosion at connector or sensor body
  • Intermittent PCM input circuit fault (rare)

Symptoms

  • Malfunction Indicator Lamp (MIL/CEL) illuminated or flashing
  • Intermittent or erratic transmission shifting (hunting between gears)
  • Delayed engagement or harsh shifts during intermittent fault
  • Transmission may enter limp mode intermittently
  • Stored/trending intermittent DTCs for pressure circuits

What to check

  • Retrieve freeze frame data and readiness status with a scan tool; note conditions when the code set
  • Scan for additional transmission or PCM codes that may indicate related electrical faults
  • Visually inspect sensor B connector and wiring for corrosion, bent pins, breaks, or pin damage
  • Back-probe the sensor connector and check for stable reference voltage (typically ~5 V) and good ground
  • Monitor live signal voltage (or switch state) while key on and during engine/transmission operation and while moving harness/connector
  • Wiggle test wiring and connector while watching live data to reproduce intermittent

Signal parameters

  • Reference supply: typically ~5.0 V (key ON). Confirm vehicle-specific value
  • Signal output: 0–5 V analog for pressure sensors (varies with transmission fluid pressure) or logic-level open/closed for switch types
  • Intermittent fault often appears as rapid voltage dropouts to 0 V or sudden jumps toward battery voltage
  • Expected signal should be stable for a given engine/transmission condition; check live data for abnormal fluctuation

Diagnostic algorithm

  1. Verify code: connect scan tool, record freeze frame and live data, confirm P0849 is current or pending.
  2. Visual inspection: examine wiring harness and connector for damage, corrosion, tightness and signs of fluid intrusion; repair obvious damage.
  3. Connector check: disconnect harness, inspect pins for corrosion/bending; clean and apply dielectric grease if minor corrosion is found.
  4. Backprobe and measure: with harness connected and ignition ON (engine OFF or as required), backprobe reference, signal and ground. Confirm stable reference voltage and good ground.
  5. Wiggle test: with live data displayed, gently move wiring and connector to try to reproduce intermittent changes; if reproduced, repair or replace wiring/connector.
  6. Continuity/resistance: with ignition OFF, check continuity between sensor ground and PCM ground, and between sensor signal terminal and PCM signal pin. Check for intermittent opens with a multimeter or scope.
  7. Load/oscilloscope test: if available, use an oscilloscope to capture transient signal dropouts, spikes or noise that a DVM might miss.
  8. Isolate sensor: if wiring checks good but intermittent persists, temporarily substitute a known-good sensor (if available) or replace the sensor and retest.
  9. After repair: clear codes, perform a test drive under conditions that previously set the code, and monitor live data to confirm the fault does not return.
  10. PCM check: only consider PCM repair/reflash or replacement after all wiring and sensor causes are eliminated and intermittent persists tied to PCM input.

Likely causes

  • Corroded or loose connector at the pressure sensor
  • Broken or chafed wiring where it flexes (intermittent break)
  • Contaminated or water-damaged sensor connector
  • Faulty pressure sensor/switch B

Fault status

⚠️ Status
Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent — PCM detected an intermittent or unstable signal from the transmission fluid pressure sensor/switch B circuit.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

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Code

P0849

MERCEDES-BENZ P — Powertrain

Trans Fluid Press Sensor/Switch B CKT Intermittent

Brand: MERCEDES-BENZ
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Corroded, loose, damaged or disconnected sensor connector or wiring harness (intermittent contact)
  • Damaged insulation causing intermittent short to voltage or ground
  • Faulty transmission fluid pressure sensor/switch B (internal intermittent)
  • Intermittent poor ground or reference voltage to the sensor
  • Water intrusion or corrosion at connector or sensor body
  • Intermittent PCM input circuit fault (rare)

Symptoms

  • Malfunction Indicator Lamp (MIL/CEL) illuminated or flashing
  • Intermittent or erratic transmission shifting (hunting between gears)
  • Delayed engagement or harsh shifts during intermittent fault
  • Transmission may enter limp mode intermittently
  • Stored/trending intermittent DTCs for pressure circuits

What to check

  • Retrieve freeze frame data and readiness status with a scan tool; note conditions when the code set
  • Scan for additional transmission or PCM codes that may indicate related electrical faults
  • Visually inspect sensor B connector and wiring for corrosion, bent pins, breaks, or pin damage
  • Back-probe the sensor connector and check for stable reference voltage (typically ~5 V) and good ground
  • Monitor live signal voltage (or switch state) while key on and during engine/transmission operation and while moving harness/connector
  • Wiggle test wiring and connector while watching live data to reproduce intermittent

Signal parameters

  • Reference supply: typically ~5.0 V (key ON). Confirm vehicle-specific value
  • Signal output: 0–5 V analog for pressure sensors (varies with transmission fluid pressure) or logic-level open/closed for switch types
  • Intermittent fault often appears as rapid voltage dropouts to 0 V or sudden jumps toward battery voltage
  • Expected signal should be stable for a given engine/transmission condition; check live data for abnormal fluctuation

Diagnostic algorithm

  1. Verify code: connect scan tool, record freeze frame and live data, confirm P0849 is current or pending.
  2. Visual inspection: examine wiring harness and connector for damage, corrosion, tightness and signs of fluid intrusion; repair obvious damage.
  3. Connector check: disconnect harness, inspect pins for corrosion/bending; clean and apply dielectric grease if minor corrosion is found.
  4. Backprobe and measure: with harness connected and ignition ON (engine OFF or as required), backprobe reference, signal and ground. Confirm stable reference voltage and good ground.
  5. Wiggle test: with live data displayed, gently move wiring and connector to try to reproduce intermittent changes; if reproduced, repair or replace wiring/connector.
  6. Continuity/resistance: with ignition OFF, check continuity between sensor ground and PCM ground, and between sensor signal terminal and PCM signal pin. Check for intermittent opens with a multimeter or scope.
  7. Load/oscilloscope test: if available, use an oscilloscope to capture transient signal dropouts, spikes or noise that a DVM might miss.
  8. Isolate sensor: if wiring checks good but intermittent persists, temporarily substitute a known-good sensor (if available) or replace the sensor and retest.
  9. After repair: clear codes, perform a test drive under conditions that previously set the code, and monitor live data to confirm the fault does not return.
  10. PCM check: only consider PCM repair/reflash or replacement after all wiring and sensor causes are eliminated and intermittent persists tied to PCM input.

Likely causes

  • Corroded or loose connector at the pressure sensor
  • Broken or chafed wiring where it flexes (intermittent break)
  • Contaminated or water-damaged sensor connector
  • Faulty pressure sensor/switch B

Fault status

⚠️ Status
Transmission Fluid Pressure Sensor/Switch B Circuit Intermittent — PCM detected an intermittent or unstable signal from the transmission fluid pressure sensor/switch B circuit.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P0840 P0842 P0848 P0850
856

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

MERCEDES-BENZ

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