What Does B1859 Mean?
This code indicates the climate control module has detected an open circuit in the A/C pressure switch monitoring circuit. The A/C pressure switch is a safety device that monitors refrigerant pressure and signals the control module to cycle the compressor clutch on/off. An open circuit means the module is not receiving any signal from the pressure switch, preventing proper A/C system operation and compressor protection.
Common Causes
45%
Faulty A/C pressure switch with internal open circuit or corroded terminals
30%
Damaged, chafed, or broken wiring between climate control module and A/C pressure switch
15%
Corroded or loose connector at A/C pressure switch or climate control module
10%
Failed climate control module with internal circuit fault
Diagnostic Steps
1
Step 1: Perform visual inspection of A/C pressure switch connector and wiring harness for visible damage, corrosion, loose connections, or chafing against sharp edges. Check connector terminal condition and ensure proper seating.
2
Step 2: Using a digital multimeter, check for continuity across the A/C pressure switch terminals with connector disconnected. Should show continuity when system is charged (typically less than 5 ohms resistance). No continuity indicates failed switch.
3
Step 3: With ignition on and A/C pressure switch disconnected, backprobe the harness connector at the switch to verify reference voltage from climate control module (typically 5V or 12V depending on manufacturer). No voltage indicates wiring fault or module issue.
4
Step 4: Test wiring continuity between A/C pressure switch connector and climate control module connector using wiring diagram. Check both signal and ground circuits for opens or excessive resistance (should be less than 1 ohm).
5
Step 5: If wiring and connections are intact, replace A/C pressure switch and verify proper refrigerant charge levels. Clear codes and test A/C operation through multiple on/off cycles.
6
Step 6: If fault persists after switch replacement and wiring verification, test climate control module inputs with oscilloscope or scan tool live data to confirm module functionality, replace module if internal fault confirmed.
Estimated Repair Cost
$100 - $450
Parts + labor, varies by vehicle and location
This code indicates the climate control module has detected an open circuit in the A/C pressure switch monitoring circuit. The A/C pressure switch is a safety device that monitors refrigerant pressure and signals the control module to cycle the compressor clutch on/off. An open circuit means the mod...
The most common cause of B1859 (Climate Control A/C Pressure Switch Circuit Open) is: Faulty A/C pressure switch with internal open circuit or corroded terminals
Typical repair costs for B1859 range from $100 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to drive, but A/C system will not function properly and compressor may not engage or may stay engaged continuously risking compressor damage. Repair within a few weeks to prevent A/C system damage and restore climate control functionality.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1859 to identify the root cause.
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Quick Info
Category
Body
System
Climate Control System
Difficulty
Type
Manufacturer
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