Code
B1257
ALFA ROMEO
B — Body
External air temperature sensor short to ground
Views:
UK: 7
EN: 9
RU: 9
AI status
Completed
Completed
100%
Causes
- Short circuit in the sensor signal wire to ground
- Damaged or chafed wiring harness contacting chassis ground
- Corroded or bent connector pins causing low-resistance path to ground
- Failed ambient/external air temperature sensor (internal short)
- Water ingress in connector or sensor body
- Incorrect installation or aftermarket sensor with wrong wiring
Symptoms
- Ambient temperature display shows incorrect or fixed very low value
- Climate control/AC automatic functions behaving incorrectly
- Rear or exterior temperature readout erratic or fluctuating
- Associated HVAC or body control warning light or message (depends on vehicle)
- DTC/BMS stored and possibly preventing certain climate features
What to check
- Retrieve codes and freeze-frame data with a scan tool; confirm B1257 is current
- Monitor live ambient temperature sensor reading and compare to actual ambient temp
- Visually inspect sensor, connector and wiring for damage, corrosion, or water
- Back-probe sensor connector and measure signal voltage with key on
- Check continuity from sensor signal pin to chassis ground with connector disconnected
- Measure sensor resistance at ambient and compare to expected thermistor behavior (resistance changes with temperature)
Signal parameters
- Type: typically NTC thermistor (resistance decreases as temperature rises)
- Expected behavior: signal value should change smoothly with ambient temperature
- Short-to-ground symptom: signal voltage near 0 V or resistance reading indicating low-ohm path to ground
- Open-circuit symptom (for reference): high resistance or infinite ohms
Diagnostic algorithm
- Confirm code: read stored/active B1257 and note freeze-frame/live data.
- Observe ambient temp reading on scan tool and compare to actual outside temperature (thermometer).
- Visually inspect sensor location (usually front bumper area or mirror), connector and harness for obvious damage or moisture.
- With ignition ON, back-probe the sensor signal wire and measure voltage. If near 0 V, suspect short to ground.
- Disconnect sensor from harness. Measure resistance from signal pin to chassis ground; a low value indicates a short in the harness. Also measure sensor resistance to verify sensor integrity.
- Perform wiggle test on harness while monitoring signal for intermittent shorts; repair any chafing or pin damage found.
- If harness wiring is intact, replace the sensor and re-test. If replacement sensor still shows short, trace and repair wiring back to the control module.
- After repair or replacement, clear codes, perform functional test and verify that ambient temperature reading responds correctly and code does not return.
- Check for Technical Service Bulletins (TSBs) or manufacturer-specific wiring diagrams if root cause unclear.
Likely causes
- Broken insulation where the signal wire rubs against a metal edge
- Connector corrosion allowing the signal to bypass into ground
- Sensor element internally shorted due to moisture or manufacturing failure
- Recent repair or work in the area that disturbed the harness or connector
Fault status
Status
External air temperature sensor circuit short to ground — ambient temperature sensor signal is being pulled to ground. Inspect sensor, connector and wiring for short to chassis ground.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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