What Does B1585 Mean?
This code indicates the Body Control Module (BCM) has detected a short to battery voltage in the dimmer panel decrease input circuit. This circuit is responsible for the dashboard instrument panel dimming function, specifically the signal that reduces brightness when the dimmer switch is activated. The BCM detected an abnormally high voltage (battery voltage) on this circuit when it should see a ground signal or lower voltage during normal operation.
Common Causes
45%
Damaged or chafed wiring harness causing the dimmer decrease circuit to contact battery positive voltage
30%
Faulty dimmer switch with internal short to power causing constant battery voltage on decrease circuit
15%
Shorted BCM internal circuitry on the dimmer decrease input channel
10%
Corroded or moisture-damaged connector pins creating unintended voltage path at dimmer switch or BCM connector
Diagnostic Steps
1
Step 1: Perform visual inspection of the dimmer switch assembly and associated wiring harness for obvious damage, chafing, or pinch points. Check all accessible connectors for corrosion, bent pins, or signs of moisture intrusion.
2
Step 2: Disconnect the dimmer switch connector and using a digital multimeter, measure voltage on the dimmer decrease circuit at the BCM side with ignition on. Reading should be approximately 5V or less (pull-up voltage). If battery voltage is present with switch disconnected, wiring harness is shorted to power.
3
Step 3: With dimmer switch still disconnected, measure resistance between the dimmer decrease circuit wire and battery positive. Should read open circuit (infinite resistance). Low resistance indicates wiring short to battery.
4
Step 4: If wiring tests normal, reconnect harness and backprobe the dimmer switch connector while operating the dimmer control. Voltage should decrease smoothly when dimming down. Constant battery voltage indicates failed dimmer switch.
5
Step 5: Check BCM connector terminals for damage, corrosion, or pushed-back pins that could cause circuit issues. Clean and repair as needed.
6
Step 6: If all wiring and switch tests are normal, suspect internal BCM fault. Clear codes, perform BCM relearn procedures if applicable, and retest. Persistent code with no external faults indicates BCM replacement needed.
Estimated Repair Cost
$100 - $600
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a short to battery voltage in the dimmer panel decrease input circuit. This circuit is responsible for the dashboard instrument panel dimming function, specifically the signal that reduces brightness when the dimmer switch is activated. ...
The most common cause of B1585 (Dim Panel Decrease Input Circuit Short To Battery) is: Damaged or chafed wiring harness causing the dimmer decrease circuit to contact battery positive voltage
Typical repair costs for B1585 range from $100 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving as this only affects instrument panel dimming function and does not impact vehicle operation or safety systems. Repair at convenience, though inability to dim dashboard lights may cause night driving discomfort.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1585 to identify the root cause.
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Quick Info
Category
Body
System
Body Electrical System - Instrument Panel Lighting
Difficulty
Type
Manufacturer
Recommended Tools
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