B1591 — Autolamp Delay Decrease Circuit Failure #

Note: The official description of DTC B1591 may differ depending on the vehicle manufacturer (OEM). Always check the OEM service manual for your specific vehicle.

• B1591: Autolamp Delay Decrease Circuit Failure (Ford, Lincoln, Mercury)
• B1591: Autolamp Off Delay Circuit Low (Mazda)
• B1591: Headlamp Auto-Off Timer Circuit Fault (Jaguar, Land Rover)

Summary of DTC B1591 #

The Diagnostic Trouble Code B1591 indicates an issue with the autolamp delay decrease circuit. This circuit controls how long your vehicle’s automatic headlights stay on after you exit or lock the car. When the system detects a malfunction—such as abnormal voltage or an open/short circuit—it sets this code. While this fault does not typically affect basic drivability, it can impact the convenience and functionality of your vehicle’s lighting system. If left untreated, it could reduce visibility when entering or leaving your vehicle at night.

Common Symptoms for DTC B1591 #

Drivers usually notice changes in how the automatic headlights behave. You may observe the headlights staying on too long or turning off too quickly after you exit the vehicle. Sometimes, the autolamp function may stop working altogether. In rare cases, a warning light may appear on the dashboard, but not all vehicles display this.

• The headlights stay on longer than normal after shutting off the engine
• The headlights shut off immediately after turning off the ignition
• Autolamp feature (automatic headlight delay) does not operate
• Inoperative headlamp auto-off function
• Occasional dashboard warning or message (varies by OEM)

Probable Causes for DTC B1591 (in order of likelihood) #

Several factors can trigger a B1591. Most often, the root cause lies with a circuit fault or a sensor malfunction. Below is a ranked list of the most common culprits, from most to least likely:

1. Faulty autolamp delay relay or control module
2. Wiring issues: open, shorted, or corroded wires in the autolamp delay circuit
3. Bad headlight switch or autolamp sensor (ambient light sensor)
4. Corroded, loose, or damaged connectors related to the circuit
5. Blown fuse or poor ground connection in the autolamp system
6. Software or configuration issue in the Body Control Module (BCM)

Step-by-Step Diagnostic Process for B1591 #

A structured approach is essential when diagnosing B1591. Modern vehicles use interconnected modules, so patience and careful inspection will save you time and frustration. Here’s a proven step-by-step method:

• 1. Scan for codes: Use a professional-grade scan tool to confirm B1591 and check for related codes (especially BCM or lighting system codes).
• 2. Visual inspection: Examine the autolamp relay, headlight switch, and all visible wiring for signs of damage, corrosion, or loose connections.
• 3. Check fuses and relays: Test all fuses and relays associated with the autolamp circuit. Replace any faulty components.
• 4. Test the ambient light sensor: Use a multimeter to verify correct operation and signal output of the ambient light/autolamp sensor.
• 5. Inspect wiring harness: Using wiring diagrams, check the continuity and resistance in the delay decrease circuit between the BCM, sensor, and switch.
• 6. Test the BCM: If all hardware checks out, use the scan tool’s live data to monitor BCM inputs and outputs related to the autolamp system.
• 7. Clear codes and retest: After repairs, clear all DTCs and verify proper autolamp operation during several ignition cycles.

Possible Repairs for B1591 #

Depending on the fault found during diagnosis, several repairs may resolve B1591. Always address the root cause, not just the symptom, for a lasting repair. Here are the most common fixes:

• Replace a faulty autolamp relay or headlight switch
• Repair or replace damaged wiring and connectors in the autolamp delay circuit
• Replace a failed ambient light sensor
• Clean or secure corroded/loose connectors
• Replace blown fuses or repair poor ground points in the lighting system
• Update BCM software or reprogram configuration (if advised by OEM)

Is It Safe to Drive with Code B1591? #

Yes—with limitations. B1591 does not affect critical safety systems or engine performance. However, your automatic headlight delay feature may not work correctly, which can be inconvenient or reduce visibility in dark areas after parking. If you often park in poorly lit locations or rely on your autolamp system, have it checked soon. Otherwise, you can continue driving, but schedule a repair at your earliest convenience.

Vehicles Most Commonly Affected by B1591 #

Based on service data and technical bulletins, B1591 is most frequently reported in vehicles equipped with autolamp (automatic headlight delay) systems, especially from the following manufacturers:

• Ford (F-150, Explorer, Taurus, Expedition, Focus, Fusion, Edge)
• Lincoln (MKZ, Navigator, Town Car)
• Mercury (Grand Marquis, Mountaineer, Sable)
• Mazda (select models with autolamp feature)
• Jaguar and Land Rover (late-model vehicles with auto headlamps)

Common Mistakes & Best Practices with B1591 #

When troubleshooting B1591, certain missteps can prolong repairs or cause misdiagnosis. To save time and money, avoid these errors and follow proven practices:

• Ignoring a thorough visual inspection—damaged wiring or connectors are often missed.
• Replacing the BCM or relay without first checking the sensor and wiring.
• Overlooking fuse or ground issues—always verify basics before replacing parts.
• Not consulting updated OEM bulletins or wiring diagrams for model-specific quirks.
• Failing to clear codes and retest after repairs.

Quick FAQ for DTC B1591 #

Still have questions? Here are quick answers to the most common concerns regarding B1591:

• Q: Will B1591 drain my battery?
  A: Not directly. However, if the autolamp system fails to turn off the headlights, prolonged illumination could drain the battery if left unaddressed.
• Q: Can I reset B1591 by disconnecting the battery?
  A: Disconnecting the battery may temporarily clear the code, but if the underlying issue remains, the code and symptoms will return. Proper diagnosis is recommended.
• Q: Is this an urgent repair?
  A: No, unless you rely heavily on the autolamp feature for nighttime safety. Schedule a repair at your earliest convenience to restore full functionality.