HVAC Temperature Differential Sensor Circuit Failure
The B1849 code indicates a fault in the HVAC temperature differential detection circuit. This circuit monitors the temperature difference between the air conditioning output and the cabin interior to regulate automatic HVAC operation. The climate control module (FCIM or equivalent) detects an out-of-range signal, open circuit, or short circuit in the differential temperature sensor or its wiring, preventing proper automatic temperature regulation.
1. Commonly Affected Makes
2. Most Common Causes
- Damaged or disconnected HVAC differential temperature sensor
- Wiring shorted to ground or battery voltage
- Open circuit in the differential temperature sensor harness
- Faulty climate control module (FCIM)
- Corroded connectors or damaged pins in the sensor circuit
3. Symptoms You Will Notice
- Automatic climate control fails to maintain set temperature
- HVAC system may blow only hot or only cold air
4. Step-by-Step Diagnosis
Scan for B1849 and any related HVAC codes
Visually inspect the differential temperature sensor and its connector for damage or corrosion
Check sensor resistance and compare to manufacturer specifications
Test wiring for shorts, opens, or high resistance between sensor and FCIM
If sensor and wiring are good, suspect a faulty FCIM module
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the automatic climate control will not function properly, which may affect comfort.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, depending on whether the sensor, wiring, or FCIM module needs replacement.