Code
B1230
MERCEDES-BENZ
B — Body
Evaporator temperature sensor (B10/6)
Views:
UK: 17
EN: 30
RU: 26
AI status
Completed
Completed
100%
Causes
- Failed/shorted/open evaporator temperature sensor (thermistor/NTC)
- Damaged, corroded or disconnected connector or wiring (open, short to ground or +V)
- Poor sensor mounting or contaminated sensor causing incorrect reading
- HVAC control module (BMS or climate control unit) fault or internal processing error
- Intermittent fault due to vibration or water ingress
Symptoms
- DTC B1230 stored and possibly HVAC or service warning message on instrument cluster
- Automatic A/C may cycle poorly or shut down to prevent evaporator freeze
- Incorrect cabin temperature control or poor climate control performance
- Evaporator icing or abnormal frost build-up (intermittent)
- Blower or A/C compressor cycling unexpectedly
What to check
- Scan for DTCs and freeze frame data; note any related HVAC codes
- Visual inspection of sensor connector, pins and wiring harness for corrosion, damage or water ingress
- Check that sensor is correctly seated in evaporator housing and free of debris
- Measure sensor resistance at ambient temperature (with sensor disconnected) and compare to OEM specification
- Backprobe sensor connector with system powered and measure reference voltage and signal voltage to HVAC control
- Perform continuity and short-to-ground/short-to-V checks on the harness
Signal parameters
- Sensor type: typically an NTC thermistor — resistance decreases as temperature rises
- Typical reference: sensor uses a reference supply (commonly 5 V) and provides a voltage signal (~0.5–4.5 V) to the HVAC control — consult OEM data for exact values
- Resistance at ~20–25 °C typically in the low kΩ range (varies by design) — verify against Mercedes-Benz specification
- Open-circuit: infinite or very high resistance; short-circuit: near 0 Ω or signal stuck to supply/ground
- Expected signal should change steadily with temperature (no sudden jumps or drops)
Diagnostic algorithm
- Retrieve all HVAC-related DTCs and live data using a capable Mercedes diagnostic tool. Record sensor voltage/resistance and ambient temperatures shown.
- Perform a visual inspection: check the sensor connector and wiring along the route for pin corrosion, chafing, water damage or loose pins. Repair as needed.
- With ignition off, disconnect the evaporator temperature sensor connector. Measure sensor resistance across its terminals at ambient temperature and compare to OEM specification. If out of range, replace sensor.
- With connector connected and ignition ON (HVAC powered), backprobe the signal and reference wires. Verify correct reference voltage (usually ~5 V) and that signal voltage changes when you change local temperature (hand/ice pack near sensor).
- Check for continuity between sensor ground and vehicle chassis ground; check for shorts to power or ground on the signal wire.
- If wiring and sensor check good, swap in a known-good sensor or temporarily bridge with a calibrated resistor to confirm HVAC module response.
- If sensor and harness are good but fault persists, test or update the HVAC control module software and check for module faults. Follow Mercedes TSBs and repair procedures.
- Clear DTCs and perform road/functional test of climate system to confirm fault does not return and that evaporator temperature readings respond correctly.
Likely causes
- Connector corrosion or loose terminal at the sensor
- Wiring short to ground or open circuit between sensor and HVAC control unit
- Failed thermistor element inside the evaporator sensor
- Sensor contaminated with moisture/foam debris or not seated in evaporator housing
Fault status
Status
Evaporator temperature sensor circuit fault detected (open/short/implausible). HVAC control module has logged B1230 and may limit A/C operation to prevent evaporator freeze or damage.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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