Code
B1251
FIAT
B — Body
Cabin temperature sensor interrupted
Views:
UK: 1
EN: 12
RU: 2
AI status
Completed
Completed
100%
Causes
- Open or broken wiring between the cabin temperature sensor and HVAC/BCM
- Disconnected, corroded or damaged sensor connector
- Failed cabin temperature (NTC) sensor
- Poor ground or supply to the sensor circuit
- Intermittent connection due to damaged insulation or terminal
Symptoms
- Automatic climate control fails to regulate cabin temperature correctly
- Incorrect or missing interior temperature reading on HVAC display
- HVAC system runs at incorrect fan/actuator states or goes to default mode
- DTC stored in HVAC or body control module and possible MIL in some vehicles
- Intermittent HVAC behavior or warning message about sensor
What to check
- Read and record all stored HVAC/body codes and freeze frame data with a compatible scan tool
- Verify live cabin temperature sensor value on scan tool (look for open/erratic reading)
- Visual inspection of sensor location, connector and wiring for damage or corrosion
- Backprobe connector and check for 5V (or reference) supply and ground at the sensor connector
- Measure sensor resistance at known ambient temperature and compare to specification
- Perform wiggle test on harness while monitoring live data to reproduce fault
Signal parameters
- Typical cabin temperature sensor is an NTC thermistor: approx. 10 kΩ at 25°C (value varies by manufacturer)
- Resistance decreases as temperature rises (NTC). Expect ~30–40 kΩ at cold (
- Open circuit: very high resistance (>100 kΩ) or OL; short: near 0 Ω
- If sensor uses a voltage output, expect roughly 0.5–4.5 V across operating range (depending on module pull‑up/reference)
- Reference supply commonly 5 V or switched reference from HVAC module; ground should be vehicle chassis or module ground
Diagnostic algorithm
- Connect a diagnostic scan tool, read and record DTCs and live cabin temp value. Note freeze frame data if present.
- Visually inspect the sensor (usually behind dash or in HVAC plenum) and its connector for damage, moisture or corrosion. Repair any visible problems.
- Backprobe the sensor connector with ignition ON and measure reference voltage and ground continuity. If no reference/ground, inspect wiring to the HVAC or body control module.
- Remove the sensor and measure resistance at ambient. Compare to manufacturer spec or typical NTC curve. If reading is open (OL) the sensor is likely failed.
- If resistance is out of spec, check continuity between sensor connector and control module pins. Repair any open/shorts found in the harness. Perform wiggle test while monitoring for intermittent faults.
- If wiring and connectors are good but readings are incorrect, replace the cabin temperature sensor and retest.
- After repair, clear codes, run vehicle to normal operating conditions and verify proper live sensor readings and HVAC operation. Confirm code does not return.
- If problem persists with correct sensor and wiring, inspect or test the HVAC control module for internal faults or intermittent connector issues.
Likely causes
- Wire harness chafing or broken conductor near dash or sensor mounting
- Connector pins pushed out, bent or corroded at sensor or control module
- Sensor element failed (open resistance)
- Previous repairs with improperly routed wiring or missing grommet
Fault status
Status
Cabin temperature sensor circuit interrupted — open or lost signal to HVAC control.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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