Autolamp Delay Decrease Circuit Failure
Your vehicle's automatic headlight delay system isn't working properly—the circuit that controls how long your lights stay on after you turn off the ignition is failing. It's like a dimmer switch that's stuck and won't adjust correctly.
The ECU monitors the autolamp delay circuit's voltage and resistance to ensure the headlight relay can energize and de-energize within acceptable timing windows. It compares actual circuit response times against programmed delay thresholds to detect open circuits, shorts, or component degradation.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Delay Circuit Voltage | 12-14V when active | Below 8V or shorted to ground |
| Relay Response Time | 0.5-2 seconds | No response or exceeds 3 seconds |
Code B1591 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.
Once the fault is repaired, B1591 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.