B1257
BodyAir Temperature External Sensor Circuit Short To Ground
The external (ambient) air temperature sensor circuit has a fault, the sensor that reads outside air temperature for the dashboard display and air conditioning system. The exact meaning varies by manufacturer.
ⓘ Information only. This page provides general educational information about fault code B1257. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does B1257 mean?
B1257 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Air Temperature External Sensor Circuit Short To Ground.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Outside temperature reading on the dashboard shows nonsense values (-40°C, 120°C, or stuck at one number)
- • Air conditioning may behave oddly, struggling to cool or producing wrong fan speeds
- • Engine warning light on some manufacturers (Hyundai, Kia)
- • Auto-wiper or auto-headlight features may misbehave on cars that share the sensor
Possible causes
Causes commonly associated with B1257, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Sensor itself failed, common after a stone strike (most live behind the front grille)
- 2. Connector unplugged or corroded, the sensor is in a harsh weather position
- 3. Wiring damaged, often where it routes through the bumper
- 4. On Ford, Mazda, and Kia: short circuit to ground in the wiring (matches the SAE definition)
- 5. On Hyundai: AQS sensor open circuit (different fault but same code)
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Locate the sensor, almost always at the front of the car behind the grille or in the front bumper. Check the connector first
- 2. Test the sensor's resistance against ambient temperature, typical reading is around 2,000-3,000 ohms at room temperature, climbing as it gets colder
- 3. Check wiring continuity back to the climate or body control module
- 4. Compare live data against actual ambient, if the sensor reads -40°C, the circuit is open
- 5. If the sensor is fine and wiring is good, the receiving module (BCM or HVAC) is the next suspect
Common questions about B1257
Why is the dashboard showing -40°C? +
That's the default value the system displays when the sensor circuit reads as open or short-to-ground. Once the underlying fault is fixed, the temperature display returns to normal within seconds. Don't bother resetting anything yourself, the dash reads the sensor live.
Will this fail my MOT? +
An ambient temperature fault by itself doesn't fail MOT. If the engine warning light is on at the time of test, that does fail MOT for cars from 2018. Get the underlying fault fixed and the light should go out within a few drive cycles.
Why does my air con behave weirdly? +
Modern climate control uses ambient temperature to decide things like recirculation, defrost behaviour, and fan speed strategy. With a faulty reading, the system makes poor decisions, you might get cold air blown when you wanted heat, or windows fogging up that should be clearing.
Manufacturer-specific notes
Some manufacturers publish additional definitions or variations for this code. Always cross-check with your vehicle's own service documentation.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →