B1231
AC Evaporator Temperature Sensor Circuit Open / Acceleration Threshold Exceeded
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Code B1231 is a manufacturer-specific body code that, on Ford vehicles, typically indicates an open circuit in the AC evaporator temperature sensor. The evaporator temperature sensor monitors how cold the evaporator core gets and is essential for proper AC operation—it prevents the evaporator from freezing over and helps regulate cabin temperature.
When this sensor circuit is open, the HVAC control module loses its ability to monitor evaporator temperature. As a safety measure, the system may disable the AC compressor entirely to prevent evaporator freeze-up, or it may allow the compressor to run but cycle it erratically. The result is typically warm or inconsistent air from the vents when you expect cold air. In vehicles with automatic climate control, the system may struggle to maintain the set temperature.
The evaporator temperature sensor itself is an inexpensive part (typically $15–$50), but access can be challenging since it's usually located inside the HVAC box behind the dashboard. On some vehicles, removing the glove box provides access; on others, significant dashboard disassembly is required. If you're handy and your vehicle has easy access to the sensor, this can be a DIY repair. Otherwise, professional repair runs $150–$500, with much of the cost being labor for dashboard access.
Severity
Symptoms
- •Air conditioning blows warm air or doesn't cool effectively
- •AC compressor may not engage
- •Inconsistent cabin temperatures with automatic climate control
- •AC system cycles on and off erratically
- •Dashboard may not display correct AC temperature
- •Defrost mode may not function properly
Likely Causes
The sensor mounted on or near the AC evaporator core can fail due to age, vibration, or moisture exposure, creating an open circuit that prevents the AC system from monitoring evaporator temperature.
The wiring from the evaporator sensor through the dashboard and HVAC box can develop breaks or loose connections, especially at connector junctions where vibration causes wear.
The evaporator area is naturally moist from condensation, which can cause the sensor connector to corrode over time, eventually creating an open circuit.
The HVAC control module that reads the evaporator sensor can develop internal faults on its input circuit, though this is less common than sensor or wiring failures.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the evaporator temperature sensor
- Inspect and repair wiring between the sensor and HVAC module
- Clean or replace corroded connectors in the sensor circuit
- Check the HVAC control module for faults
- Verify AC system refrigerant charge is correct