B1591 Autolamp Delay Decrease Circuit Fault
The B1591 code indicates a fault in the autolamp delay decrease circuit. This system controls how long the exterior lights stay on after the doors are closed, allowing the driver to see their path to the destination. The control module detects an anomaly in the circuit that reduces the programmed delay time, typically due to a problem with the autolamp delay switch or its signal circuit to the lighting module (GEM or BCM on Ford vehicles).
1. Commonly Affected Makes
2. Most Common Causes
- Faulty or worn autolamp delay switch
- Switch signal circuit shorted to ground or voltage
- Abnormal resistance in the delay decrease circuit wiring
- Corroded or damaged delay switch connector pins
- Internal failure of the GEM or BCM module
- Voltage variation in the switch sensor reference circuit
3. Symptoms You Will Notice
- Exterior lights turn off immediately or too quickly after closing doors
- Autolamp delay setting has no effect or is erratic
4. Step-by-Step Diagnosis
Scan for related codes and verify B1591 is present
Inspect the autolamp delay switch and its connector for damage or corrosion
Test the switch circuit for shorts, opens, or abnormal resistance
Check voltage supply and ground at the switch and module
If all else is normal, suspect a faulty GEM or BCM
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, it is safe to drive. The fault only affects the delayed lighting feature; the lights themselves still function normally.
How much does it cost to fix?▼
Repair costs typically range from $50 to $200 for a switch replacement, up to $500 or more if the GEM or BCM needs replacement.