B1257

Code

Other B — Body

Air Temperature External Sensor Circuit Short To Ground

Brand: Other

Type: B — Body

Views: UK: 22 EN: 48 RU: 33

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Damaged or chafed wiring contacting chassis ground
Corroded, bent or damaged sensor connector pins
Failed external air temperature sensor (NTC thermistor) with internal short
Water intrusion at connector causing short to ground
Aftermarket accessories or previous repairs that pinched/grounded the signal wire
Faulty body or HVAC control module (rare)

Symptoms

Outside/ambient temperature display shows extremely low or implausible values (very cold or - - -)
Climate control behaves incorrectly (auto mode may be affected)
DTC B1257 stored; possible related HVAC/warning message on dash
No drivability issues in most vehicles (sensor used for cabin display/climate logic)

What to check

Read and record freeze frame and live data for the external air temperature sensor with a scan tool
Visual inspection of sensor mounting, connector and wiring for damage, corrosion, water entry, or pinched sections
Unplug sensor and inspect connector terminals for corrosion, bent pins or contaminant bridging
Check continuity from sensor signal pin to chassis ground; verify whether the circuit is shorted
Backprobe signal circuit and verify reference/pull-up voltage with ignition on (compare connected vs disconnected)
Measure sensor resistance at ambient temperature (if accessible) and compare to expected values from service data

Signal parameters

Sensor type: typically an NTC thermistor (resistance varies with temperature)
Typical resistance at ~25°C: manufacturer-specific (commonly in range ~1 kΩ to 50 kΩ). Refer to service manual for exact values
Control module measures voltage across sensor via a pull-up/reference (typical signal voltage range ~0.5–4.5 V depending on temperature and vehicle design)
Short-to-ground symptom: signal voltage near 0 V or extremely low resistance to ground (continuity to chassis)
Open-circuit symptom (related code): very high voltage/open circuit or out-of-range resistance

Diagnostic algorithm

Verify: Use a scan tool to confirm B1257 is current or stored and note live sensor value, freeze frame and MIL status.
Visual: Inspect sensor and connector for corrosion, bent pins, water ingress, damage or loose mounting. Repair obvious issues.
Check connector off: Disconnect the sensor. With ignition ON (engine off), measure voltage at the signal terminal — there should typically be a pull-up/reference voltage present. If voltage is near 0 V with sensor disconnected, the harness is shorted to ground or module output is faulty.
Continuity test: With ignition off, measure continuity between the sensor signal wire and chassis ground. Continuity indicates a short to ground. Isolate sections of harness to locate short (unplug intermediate connectors, use harness routing to narrow location).
Sensor resistance: Measure the sensor resistance directly (at the sensor) and compare to expected resistance table for ambient temperature. If the sensor reads near 0 Ω, it may be internally shorted.
Wiggle/road test: If no fault found visually, backprobe the circuit and wiggle the wiring while monitoring live data to reproduce an intermittent short. Check areas where the harness passes through grommets or near metal edges.
Repair: Repair or replace damaged wiring, connectors, or sensor as required. Use proper insulation, shrink tubing, or conduit to prevent future chafing. Replace the sensor if it has failed.
Module check: If harness and sensor test good, consult vehicle wiring diagrams and test the control module output/pull-up circuit per factory procedures; replace module only after confirming module fault.
Verify: Clear codes, retest sensor operation and drive or cycle ignition to confirm the code does not return.

Likely causes

Wiring harness chafed and shorted to chassis ground near sensor or routing point
Corroded/contaminated connector at sensor causing low resistance to ground
Failed sensor with internal short to ground
Poor or missing insulation where harness passes through body panels
Module input circuit fault (least likely)

Fault status

⚠️ Status

Air Temperature External Sensor Circuit Short To Ground — control module detected the ambient/outside air temperature sensor circuit at or near ground potential, indicating a short to chassis ground or sensor/wiring fault.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours