What Does B2559 Mean?
This code indicates the Body Control Module (BCM) detected battery voltage present on the auxiliary A/C blower motor relay control circuit when the relay should be de-energized. This suggests the relay circuit has an unwanted connection to battery positive voltage, causing the relay to remain energized or allowing current flow when it shouldn't. This typically affects vehicles equipped with rear or auxiliary climate control systems.
Common Causes
40%
Damaged or chafed wiring harness causing short to power between relay and BCM
30%
Failed auxiliary A/C blower motor relay with internal short
20%
Faulty Body Control Module with shorted relay driver circuit
10%
Corroded or damaged relay socket/connector with cross-circuit contamination
Diagnostic Steps
1
Step 1: Locate the auxiliary A/C blower motor relay in the underhood or auxiliary fuse/relay box. Visually inspect the relay, socket, and surrounding connectors for corrosion, melted terminals, or physical damage.
2
Step 2: Remove the auxiliary blower motor relay and use a digital multimeter to check for battery voltage (12-14V) on the relay control terminal from the BCM with ignition on. If voltage is present when relay is removed, wiring has a short to power.
3
Step 3: With relay removed and ignition off, perform continuity test from the BCM control wire terminal at the relay socket to battery positive. Any continuity indicates a short to power in the harness requiring trace and repair.
4
Step 4: If wiring tests good, test the relay itself by applying 12V and ground to coil terminals and checking for proper switching on contact terminals. Replace relay if it shows internal short or fails to operate correctly.
5
Step 5: Check BCM connector terminals for corrosion or pushed-back pins. If all wiring and relay test good, BCM may have internal relay driver failure requiring module replacement.
6
Step 6: After repairs, clear codes, cycle auxiliary A/C system on and off multiple times, and verify code does not return and blower operates normally under BCM command.
Estimated Repair Cost
$100 - $800
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) detected battery voltage present on the auxiliary A/C blower motor relay control circuit when the relay should be de-energized. This suggests the relay circuit has an unwanted connection to battery positive voltage, causing the relay to remain energi...
The most common cause of B2559 (Aux A/C Blower Motor Relay Circuit Short to Battery) is: Damaged or chafed wiring harness causing short to power between relay and BCM
Typical repair costs for B2559 range from $100 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving as this only affects auxiliary A/C comfort function. Repair at earliest convenience to restore rear climate control and prevent potential battery drain if relay remains energized continuously.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B2559 to identify the root cause.
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Quick Info
Category
Body
System
Auxiliary Climate Control System
Difficulty
Type
Generic (SAE)
Recommended Tools
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