Air Temperature External Sensor Circuit Short To Battery
The external air temperature sensor is sending a voltage signal that's too high, as if it's shorted directly to the vehicle's battery power. Think of it like a water pipe that's supposed to measure flow but instead has a hole spraying directly into the main water line.
The ECU monitors the external air temperature sensor's voltage signal, which should vary between 0.5V and 4.5V based on ambient temperature. When the circuit shorts to battery voltage (typically 12V+), the ECU detects an out-of-range high signal and sets the fault code. The sensor resistance should increase as temperature drops, but a short-to-battery bypasses normal sensor operation.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5V to 4.5V | >4.8V (shorted to battery) |
| Sensor Resistance | 10kΩ to 100kΩ | <1kΩ (indicates short) |
Code B1256 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, B1256 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.