Air Temperature External Sensor Circuit Open
The external air temperature sensor isn't sending a signal to the engine control unit, like a broken thermometer that won't report the outdoor temperature. The ECU can't read ambient air conditions, which affects fuel mixture and cooling fan operation.
The ECU monitors voltage from the external air temperature sensor to determine ambient conditions for adaptive fuel and cooling strategies. The sensor produces a variable voltage signal (typically 0.5-4.5V) that corresponds to temperature. An open circuit causes the signal to drop to zero or remain at an extreme value outside the expected operating range.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5 to 4.5 volts (proportional to temperature) | Below 0.1V or above 4.9V (open circuit detected) |
| Temperature Range | -40°C to +125°C (-40°F to +257°F) | Signal absent or implausible (open circuit) |
Code B1255 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.
Once the fault is repaired, B1255 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.