B1228

Code

MERCEDES-BENZ B — Body

Heater core temperature (B10/1)

Brand: MERCEDES-BENZ

Type: B — Body

Views: UK: 13 EN: 13 RU: 9

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Page language: EN

Causes

Faulty heater core temperature sensor (B10/1)
Damaged, corroded or disconnected wiring/connector to the sensor
Open or short circuit in sensor signal or supply/ground wiring
Poor ground at HVAC control module or sensor
Restricted coolant flow, clogged heater core or air in cooling circuit causing erroneous readings
Faulty HVAC control module or intermittent software/communication error

Symptoms

HVAC/heater not maintaining commanded cabin temperature
Erratic or incorrect heater temperature display or readout
Persistent B1228 fault stored in control module
Reduced or no heater output despite engine at operating temperature
Possible HVAC warning or message on instrument cluster
Intermittent HVAC operation or fluctuating temperature control

What to check

Read fault memory and freeze frame with a factory scan tool; note current and historic occurrences
Check live data: heater core temperature sensor value and compare with engine coolant temperature, ambient and cabin sensors
Visually inspect sensor connector (B10/1) and wiring for corrosion, damage, pin push-out or water intrusion
Check coolant level and condition; inspect for air in system and proper heater core flow
Backprobe sensor connector and measure signal voltage/resistance while varying coolant/engine temperature
Check for proper ground at HVAC control module and engine grounding points

Signal parameters

Sensor type: typically an NTC thermistor mounted in or near the heater core (resistance decreases as temperature rises)
Typical signal behavior: resistance decreases with increasing temperature; diagnostic module expects a smooth, plausible curve vs engine/coolant temp
Common measurement ranges (factory-specific): resistances commonly in the low hundreds to several kiloohms depending on temperature, or corresponding voltages in the approximate 0.5–4.5 V range — consult vehicle-specific data for exact values
Fault triggers: open circuit (very high resistance/OL), short to ground (very low resistance), or implausible jumps/intermittent signal

Diagnostic algorithm

Retrieve and record B1228 with a capable Mercedes scan tool; note freeze frame and related HVAC or coolant codes.
With ignition on, view live sensor value for B10/1 and compare with engine coolant temperature and cabin temperature sensors for plausibility.
Visually inspect sensor and connector for corrosion, bent pins, water entry, or physical damage; reconnect and recheck fault if found.
Measure sensor resistance at the connector (engine cold and warm) and compare to OEM specification or expected NTC behavior (resistance should smoothly change with temperature).
If resistance out of range, remove and replace sensor, then clear codes and retest.
If resistance is plausible, backprobe the signal wire at the control module to check for intermittent voltage drops, short to power/ground, and continuity to the sensor.
Check coolant level, bleed air from system, and verify heater core flow (warm-up of heater hoses and core temperature). Repair any coolant flow issues and retest.
If wiring and sensor are good but fault persists, inspect module grounds and CAN/communication lines; consider control module diagnostic or software update as directed by dealer/service information.
After repairs, clear codes and perform a road/drive cycle and re-scan to confirm the fault does not return.

Likely causes

Connector corrosion or water ingress at sensor
Broken or chafed signal wire between sensor and control module
Sensor thermistor out of tolerance (age or heat damage)
Low coolant level or trapped air preventing proper heater core temperature change
Intermittent connector pin contact causing fluctuating readings

Fault status

⚠️ Status

Stored fault: Heater core temperature sensor (B10/1) signal out of range/implausible or open/short condition detected. Monitor and diagnose sensor circuit and related coolant/heater core conditions.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours

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B1228 — Heater core temperature (B10/1)