What Does B1591 Mean?
This code indicates the Body Control Module (BCM) has detected a malfunction in the circuit that controls the autolamp delay decrease function. This system allows the headlights to remain on for a user-adjustable period after the vehicle is turned off. The BCM has detected either an open circuit, short circuit, or improper signal from the decrease switch or its associated wiring that prevents proper adjustment of the delay timer.
Common Causes
45%
Faulty autolamp delay decrease switch or worn switch contacts
30%
Open or short circuit in wiring between BCM and autolamp delay switch
15%
Corroded or loose electrical connector at switch or BCM
10%
Failed Body Control Module (BCM) internal circuitry
Diagnostic Steps
1
Step 1: Perform visual inspection of the autolamp delay switch assembly and all associated connectors for signs of physical damage, corrosion, or loose connections. Check connector terminals for proper seating and corrosion.
2
Step 2: Using a digital multimeter, check for proper voltage supply to the autolamp delay switch (typically 5V or 12V reference from BCM). With ignition on, backprobe the switch connector and verify reference voltage is present.
3
Step 3: Test the autolamp delay decrease switch operation by measuring resistance or continuity across switch terminals while activating the decrease function. Compare readings to manufacturer specifications (typically should show resistance change or continuity when pressed).
4
Step 4: Check wiring continuity between the autolamp delay switch and BCM connector using a multimeter. Test for opens, shorts to ground, or shorts to power in the circuit. Pay special attention to harness areas subject to flexing or wear.
5
Step 5: If all wiring and switch tests pass, monitor BCM input data using advanced scan tool functions while manually operating the decrease switch to verify BCM is receiving proper signal input.
6
Step 6: Clear the code, cycle the autolamp delay decrease function multiple times, and verify repair by confirming the code does not return and the delay timer adjustment functions properly.
Estimated Repair Cost
$75 - $400
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a malfunction in the circuit that controls the autolamp delay decrease function. This system allows the headlights to remain on for a user-adjustable period after the vehicle is turned off. The BCM has detected either an open circuit, sh...
The most common cause of B1591 (Autolamp Delay Decrease Circuit Failure) is: Faulty autolamp delay decrease switch or worn switch contacts
Typical repair costs for B1591 range from $75 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving as this only affects the convenience feature of adjusting the headlight delay timer after engine shutdown. The headlights will continue to function normally for safety. Repair at your convenience.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1591 to identify the root cause.
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Quick Info
Category
Body
System
Body Electrical System - Lighting Control
Difficulty
Type
Manufacturer
Recommended Tools
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