B1255

What Does Code B1255 Mean?

DTC B1255 signifies an “Air Temperature External Sensor Circuit Open” condition. This fault indicates that the Body Control Module (BCM), Powertrain Control Module (PCM), or dedicated HVAC control module has detected an electrical open circuit within the external ambient air temperature sensor’s wiring or the sensor itself. The external air temperature sensor is typically a Negative Temperature Coefficient (NTC) thermistor, meaning its electrical resistance decreases as the ambient temperature increases. The control module supplies a reference voltage (commonly 5V) through a pull-up or pull-down resistor to the sensor circuit. In an open circuit scenario, the resistance within the circuit becomes extremely high or infinite, causing the voltage signal returned to the control module to rise to or near the reference voltage. This out-of-range high voltage is interpreted by the control module as an open circuit, leading to the storage of code B1255. This subsystem primarily affects the accuracy of the external temperature display and the operational parameters of automatic climate control systems.

Common Symptoms

• Inaccurate or Non-Functional External Temperature Display: The most prevalent symptom is an incorrect temperature reading on the instrument cluster or infotainment screen, often defaulting to a fixed value such as -40°C (-40°F), a maximum/minimum value, or displaying dashes.
• Erratic or Inefficient Automatic Climate Control Operation: Vehicles equipped with automatic climate control systems may exhibit improper fan speed, air blend door positioning, or compressor engagement due to receiving erroneous ambient temperature data.
• No Visible Symptom Other Than Code: In some applications, if the sensor’s function is purely for display and not integrated into critical vehicle operations, the only indication may be the stored diagnostic trouble code without any noticeable operational changes.

What Causes the Code B1255?

• Faulty External Air Temperature Sensor: The sensor itself may have an internal electrical open circuit due to age, thermal cycling, or physical damage.
• Damaged Wiring Harness: An open circuit in the wiring connecting the external air temperature sensor to its respective control module. This can be caused by physical abrasion, cuts, pinching, or rodent damage, especially given the sensor’s often exposed location.
• Corroded Electrical Connectors: Corrosion within the sensor connector or the harness connector terminals due to moisture ingress, road salt exposure, or chemical contamination, leading to a high-resistance or open connection.
• Loose or Disconnected Connector: The sensor connector may have vibrated loose or become disconnected entirely from the sensor terminals.
• Faulty Control Module (Rare): Although uncommon, an internal defect within the BCM, HVAC control module, or PCM could erroneously generate the B1255 code despite a functional sensor and intact wiring. This is typically a last resort diagnosis.

How to Diagnose and Troubleshoot

A systematic approach is crucial for diagnosing B1255:

1. Visual Inspection: Begin by locating the external air temperature sensor, often positioned behind the front bumper, in front of the radiator, or integrated into a side mirror assembly. Visually inspect the sensor and its entire wiring harness for obvious signs of physical damage, chafing, cuts, corrosion at the connector, or loose connections. Pay close attention to areas prone to road debris impact.
2. OBD-II Scanner Live Data Analysis: Connect an advanced OBD-II scanner and access the live data stream. Monitor the “Ambient Air Temperature” (AAT) or “External Air Temperature” (EAT) parameter. An open circuit will typically manifest as a default low reading (-40°C/-40°F), an implausible high reading, or a fixed value that does not correlate with actual ambient conditions.
3. Digital Multimeter (DMM) Testing:
   • Sensor Resistance Check: Disconnect the external air temperature sensor’s electrical connector. Using a DMM set to ohms, measure the resistance directly across the two terminals of the sensor itself. An open circuit within the sensor will register as infinite resistance (often displayed as “OL” or “1.” on the DMM). Compare the measured resistance to the manufacturer’s specifications at the current ambient temperature. A functional NTC thermistor’s resistance should decrease as it warms (e.g., by holding it in your hand).
   • Wiring Continuity Test: With the sensor connector disconnected and, if feasible, the corresponding control module connector disconnected (refer to wiring diagrams), use the DMM set to ohms to check for continuity between each wire terminal at the sensor harness connector and its respective pin at the control module connector. An open circuit in the wiring will show infinite resistance (“OL”). Continuity should be near 0 ohms.
   • Short to Ground/Power Test: With the control module and sensor disconnected, check for a short to ground by placing one DMM lead on each wire terminal at the sensor harness connector and the other lead to a known good chassis ground. Both wires should show infinite resistance. Then, check for a short to power by measuring resistance between each wire terminal and a known 12V source (with ignition ON). Again, infinite resistance should be observed.
   • Reference Voltage and Ground Verification: Reconnect the control module (but keep the sensor disconnected). With the ignition ON (engine OFF), use the DMM to check for the reference voltage (typically 5V) on one pin of the sensor harness connector and a good ground on the other pin, according to the vehicle’s wiring diagram. An open circuit might still show the reference voltage, but without the sensor completing the circuit, the module cannot read the correct voltage drop.

Recommended Repairs and Solutions

Once the root cause has been identified through systematic diagnosis:

• Replace External Air Temperature Sensor: If testing confirms an internal open circuit within the sensor, replace it with a new OEM-quality or equivalent aftermarket part. Ensure proper mounting and connection.
• Repair or Replace Wiring Harness: If an open circuit is found in the wiring, repair the damaged section using industry-standard automotive wiring repair techniques, such as soldering with heat-shrink tubing or specialized crimp connectors. If the damage is extensive or involves multiple conductors, it may be more efficient to replace the entire segment of the wiring harness. Ensure the repaired section is properly routed and secured to prevent future damage.
• Clean and Secure Connectors: For issues related to corrosion or poor contact, thoroughly clean the affected electrical terminals using an electrical contact cleaner and a small brush. Apply dielectric grease to prevent future corrosion and ensure the connector is firmly seated and latched.
• Clear DTCs and Verify Repair: After any repair, use an OBD-II scanner to clear the stored B1255 code. Test drive the vehicle and monitor the external temperature display and live data parameters to confirm accurate readings and proper operation of the climate control system. Ensure the code does not return.
• Control Module Replacement (Last Resort): If all other components and wiring have been meticulously tested and verified to be functional, and the code persists, the control module itself may be faulty. This is a rare occurrence and should only be considered after exhaustive troubleshooting. Module replacement typically requires specialized programming and calibration.