Code
B1231
MERCEDES-BENZ
B — Body
ECT sensor (B11/4)
Views:
UK: 15
EN: 19
RU: 15
AI status
Completed
Completed
100%
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
- Confirm the code and note freeze-frame data. Clear codes and try to re-produce the fault under the same conditions.
- Inspect the sensor connector for corrosion, bent pins, moisture, or looseness. Repair or clean connector as needed.
- With ignition ON (engine OFF), measure reference voltage at the sensor connector; verify expected ~5 V (consult vehicle data).
- 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.
- 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.
- Check continuity of the wiring between sensor connector and the engine control module; check for short to ground or battery using an ohmmeter.
- If wiring and sensor test OK but signal is implausible, inspect engine grounds and power supply to the ECU. Repair any issues and retest.
- 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.
- 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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