HVAC Temp Sensor Circuit High Voltage
The B1851 code indicates that the HVAC control module has detected excessive voltage on the temperature differential sensor circuit, specifically a short to battery positive (B+). This sensor measures the temperature difference between two points in the HVAC system to automatically regulate cabin temperature, airflow, and A/C compressor cycling. When the circuit is shorted to B+, the signal stays above the normal operating range, preventing the module from reading valid data. This can cause erratic climate control behavior or disable automatic temperature functions.
1. Commonly Affected Makes
2. Most Common Causes
- Short to battery positive in the temperature differential sensor wiring
- Faulty HVAC temperature differential sensor with altered internal resistance
- Corroded, wet, or bridged sensor circuit connectors causing contact with battery voltage
- Internal failure in the HVAC control module misinterpreting the signal
- Excessive resistance or short in shared circuits affecting the sensor ground reference
3. Symptoms You Will Notice
- Automatic climate control not working properly
- A/C compressor cycling irregularly or not engaging
4. Step-by-Step Diagnosis
Check for technical service bulletins (TSBs) related to B1851 for your vehicle
Inspect the temperature differential sensor wiring and connectors for damage, corrosion, or shorts to B+
Test the sensor's resistance and compare to manufacturer specifications
Check for voltage at the sensor signal wire with ignition on; should not be near battery voltage
If wiring and sensor are okay, suspect a faulty HVAC control module
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the HVAC system may not function properly, affecting comfort. The vehicle remains drivable.
How much does it cost to fix?▼
Repair costs typically range from $100 to $400, depending on whether the issue is a simple wiring repair or requires replacing the sensor or HVAC control module.