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Home / DTC / B1231 — ECT sensor (B11/4)

B1231 — ECT sensor (B11/4)

Detailed page for trouble code B1231.

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Code

B1231

MERCEDES-BENZ B — Body

ECT sensor (B11/4)

Brand: MERCEDES-BENZ
Type: B — Body
AI status
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Page language: EN

Causes

  • Damaged or failed ECT sensor (B11/4)
  • Corroded, loose or damaged connector or wiring harness
  • Open or short to ground or battery voltage in the signal or reference circuit
  • Low coolant level or poor sensor contact with coolant
  • Faulty engine control module (rare)
  • Incorrect replacement sensor or wrong specification

Symptoms

  • Incorrect coolant temperature reading on dash or scan tool
  • Hard cold start, rough idle or poor driveability until warm
  • Increased fuel consumption and elevated emissions
  • Cooling fan not operating correctly or running constantly
  • Possible stored related engine codes (fuel trim, cold start)

What to check

  • Read freeze-frame data and live coolant temperature on a scan tool. Note conditions when fault set.
  • Visually inspect sensor, connector and wiring for corrosion, damage or loose pins.
  • Verify coolant level and that sensor tip is immersed in coolant (after cool-down).
  • Check fuses and related power/ground circuits for the engine control module.
  • Wiggle wiring harness while monitoring live data to check for intermittent changes.
  • Compare ECT signal against known cold and hot reference values (see signal parameters).

Signal parameters

  • Reference supply to sensor: typically 5 V (confirm with manufacturer data)
  • Signal voltage behavior: cold engine ≈ near reference voltage (around 4–5 V), hot engine ≈ low voltage (around 0.5–1 V) for an NTC-type sensor using a pull-up
  • Resistance vs temperature: NTC sensor resistance decreases as temperature rises (compare to factory chart)
  • Typical evidence of open circuit: signal pegged at or near reference voltage with no change as engine warms
  • Typical evidence of short to ground: signal near 0–0.5 V and no change with temperature

Diagnostic algorithm

  1. Confirm the code and note freeze-frame data. Clear codes and try to re-produce the fault under the same conditions.
  2. Inspect the sensor connector for corrosion, bent pins, moisture, or looseness. Repair or clean connector as needed.
  3. With ignition ON (engine OFF), measure reference voltage at the sensor connector; verify expected ~5 V (consult vehicle data).
  4. Back-probe the sensor signal pin and observe voltage with a scan tool while starting from cold and allowing engine to reach normal temperature. Verify the signal moves smoothly from high to low as the engine warms.
  5. With the sensor disconnected, measure resistance across the sensor body terminals at ambient temperature and compare to manufacturer specification or a general NTC trend (resistance falls with increasing temp). If out of spec, replace sensor.
  6. Check continuity of the wiring between sensor connector and the engine control module; check for short to ground or battery using an ohmmeter.
  7. If wiring and sensor test OK but signal is implausible, inspect engine grounds and power supply to the ECU. Repair any issues and retest.
  8. If all wiring and inputs are correct and fault persists, consider swapping or bench-testing ECU input channel per manufacturer procedure or consult dealer documentation for ECU faults.
  9. After repair or replacement, clear codes and verify normal operation and that the code does not return under the same operating conditions.

Likely causes

  • Wiring connector corrosion at the sensor
  • Broken/shorted wire between sensor and ECU
  • Sensor failed internally (NTC element out of spec)
  • Coolant not contacting sensor (low level or airlock)

Fault status

⚠️ Status
Malfunction in Engine Coolant Temperature (ECT) sensor circuit (B11/4): sensor output implausible, open or short detected.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-1.5 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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