B1859: A/C Pressure Sensor Open Circuit
The B1859 code indicates that the Body Control Module (BCM) or HVAC control module has detected an open circuit in the A/C pressure switch/transducer signal circuit. This means the reference voltage or return signal is not reaching the control module, preventing refrigerant pressure monitoring and disabling the compressor as a safety measure.
1. Commonly Affected Makes
2. Most Common Causes
- Faulty A/C pressure transducer (internal open circuit)
- Damaged or cut wiring between the sensor and BCM/HVAC module
- Sensor connector disconnected, loose, or with corroded/retracted terminals
- Short to ground or battery in the signal/reference circuit (less common for open, but possible if module interprets it as such)
- Internal failure of the BCM or HVAC control module
3. Symptoms You Will Notice
- A/C compressor not engaging
- No cold air from A/C system
- A/C system inoperative or intermittent operation
- Check Engine Light or A/C warning light on
4. Step-by-Step Diagnosis
Scan for codes and verify B1859 is present; check for other related codes.
Visually inspect the A/C pressure sensor connector and wiring for damage, corrosion, or disconnection.
Test the sensor's reference voltage (typically 5V) and ground at the connector.
Measure the sensor signal voltage with the engine running and A/C on; compare to manufacturer specs.
If signal is missing, backprobe the module connector to check for continuity; repair open circuit as needed.
If wiring and sensor are good, suspect a faulty BCM or HVAC module.
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is generally safe to drive, but the A/C will not work. The vehicle can be driven normally without risk of damage, though the lack of A/C may be uncomfortable.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, depending on the cause. A new pressure sensor costs $40-$100, plus labor. Wiring repairs are usually $100-$200. Module replacement can exceed $500.