B1691: Autolamp Delay Circuit Short to Battery
The B1691 code indicates a short to battery voltage in the Autolamp Delay circuit. This system controls how long the exterior lights stay on after the doors are closed or the vehicle is exited. The lighting control module (LCM or FEM, depending on the model) detects abnormally high voltage on the control circuit, typically caused by a short to a battery feed wire. This fault prevents proper control of the light-off delay, potentially causing the lights to stay on indefinitely or the module to shut down for protection.
1. Commonly Affected Makes
2. Most Common Causes
- Wiring in the Autolamp delay circuit shorted to a battery power wire
- Lighting Control Module (LCM) with internal shorted output transistor
- Headlamp relay with shorted coil or contacts
- Connector or harness splice with moisture/corrosion bridging circuits
- Ambient light sensor with internal failure affecting reference signal
- Aftermarket electrical accessories interfering with the circuit
3. Symptoms You Will Notice
- Exterior lights stay on continuously or for an abnormally long time
- Autolamp delay function inoperative (lights turn off immediately or not at all)
- Battery drain due to lights not turning off
4. Step-by-Step Diagnosis
Check for technical service bulletins (TSBs) related to the LCM or Autolamp system.
Inspect the Autolamp delay circuit wiring for physical damage, chafing, or shorts to battery voltage.
Test the ambient light sensor and its circuit for proper operation and voltage.
Check the headlamp relay for shorted coil or contacts.
Evaluate the LCM for internal faults by monitoring circuit voltages and performing a wiggle test.
Disconnect any aftermarket accessories that may be tied into the lighting circuit.
5. Frequently Asked Questions
Is it safe to drive?▼
Yes, but the lights may stay on and drain the battery, or the system may fail. It's best to diagnose and repair promptly to avoid a dead battery or electrical damage.
How much does it cost to fix?▼
Typically $100–$400, depending on the cause. Wiring repairs are cheaper; replacing the LCM can cost $200–$400 including labor.