B1255 — External air temperature sensor open circuit

Causes

• Open or broken sensor wiring (broken conductor, chafing, connector unplugged)
• Corroded or bent connector pins / poor pin contact
• Faulty ambient air temperature sensor (NTC thermistor open)
• Water intrusion or contamination in connector
• Short to voltage in the sensor circuit or poor ground (less common)
• Faulty body control module (rare)

Symptoms

• Ambient/outside temperature gauge or display incorrect or shows a fixed/default value
• Climate control may behave incorrectly (automatic mode less accurate)
• Heated mirror/defog features tied to ambient temp may not operate as expected
• Body control module stored fault(s) related to ambient sensor
• Possible loss of features that depend on outside temperature (e.g., remote start restrictions)

What to check

• Use a scan tool to read live ambient/outside air temperature value and freeze-frame data
• Visually inspect sensor location, connector and wiring for damage, corrosion or water ingress
• Back-probe connector with ignition on and measure sensor voltage (or resistance with ignition off) per OEM specs
• Perform a wiggle test of the harness while observing live data for intermittent changes
• Check continuity from sensor connector to the body control module connector; check for short to voltage/ground
• Swap with a known-good sensor or temporarily bridge with a resistor to confirm ECU response (if available and safe)

Signal parameters

• Typical ambient sensors are NTC thermistors (resistance decreases as temperature rises); example: ~10 kΩ at 25°C (varies by design)
• Expected resistance vs temperature should match OEM chart (consult Fiat data). Open circuit → very high/OL resistance
• Voltage at sensor circuit depends on ECU pull-up/pull-down: often ~0.5–4.5 V across normal range; open-circuit may show near 0 V or near supply (≈5 V) depending on design
• Continuity: near-infinite resistance = open circuit; short to supply/ground will show low resistance

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