What Does B1257 Mean?
This code indicates the Body Control Module (BCM) has detected a short to ground in the external ambient air temperature sensor circuit. The sensor, typically mounted in the front bumper or grille area, provides outside temperature data to the climate control system and instrument cluster. The BCM has measured voltage below the acceptable threshold, indicating the signal wire is shorted to vehicle ground or the sensor itself has an internal short.
Common Causes
45%
Damaged or chafed wiring harness in front bumper/grille area causing short to ground
30%
Failed ambient air temperature sensor with internal short to ground
15%
Corroded or moisture-damaged sensor connector creating ground path
10%
Pinched or damaged wiring during previous service or collision repair
Diagnostic Steps
1
Step 1: Perform visual inspection of ambient air temperature sensor location in front bumper/grille area, checking for physical damage, corrosion, moisture intrusion, or wire chafing against metal surfaces
2
Step 2: Disconnect the ambient air temperature sensor connector and use a digital multimeter to measure resistance between the sensor signal pin and ground; reading should be very high (>10K ohms). If near zero ohms, replace sensor
3
Step 3: With sensor disconnected, check resistance between the signal wire pin at the harness connector and known good ground; should read infinite (open circuit). If continuity exists, trace and repair short in wiring harness
4
Step 4: Inspect sensor connector terminals for corrosion, spread terminals, or moisture damage; clean with electrical contact cleaner and apply dielectric grease if needed
5
Step 5: After repairs, reconnect sensor and clear codes; perform a road test or allow vehicle to sit while monitoring live data to verify sensor reads reasonable ambient temperature and code does not return
6
Step 6: Verify instrument cluster and climate control system display correct outside temperature reading
Estimated Repair Cost
$75 - $350
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a short to ground in the external ambient air temperature sensor circuit. The sensor, typically mounted in the front bumper or grille area, provides outside temperature data to the climate control system and instrument cluster. The BCM h...
The most common cause of B1257 (Air Temperature External Sensor Circuit Short To Ground) is: Damaged or chafed wiring harness in front bumper/grille area causing short to ground
Typical repair costs for B1257 range from $75 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This code does not affect vehicle safety or driveability. The vehicle is safe to operate normally, though the climate control system may not function optimally and outside temperature display will be inaccurate or blank. Repair can be scheduled at convenience.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1257 to identify the root cause.
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Quick Info
Category
Body
System
Climate Control and Body Systems
Difficulty
Type
Manufacturer
Recommended Tools
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