B1371

What Does Code B1371 Mean?

DTC B1371 signifies an electrical malfunction within the control circuit for the Illuminated Entry Relay. In most automotive architectures, this code is set by the Body Control Module (BCM) or a similar integrated control module responsible for interior and exterior lighting functions. The illuminated entry system typically activates interior courtesy lights (dome lights, footwell lights, sometimes puddle lights) upon unlocking the vehicle with a remote or key, or when a door is opened. The BCM monitors the electrical characteristics of the relay’s control circuit – specifically, the voltage and current flow when it attempts to energize the relay coil. If the BCM detects an open circuit (no current flow when commanded), a short to ground, a short to voltage, or an abnormally high resistance within this circuit that prevents proper relay operation, it will set the B1371 code. This indicates that the BCM cannot reliably command the relay to its ON or OFF state, directly impacting the functionality of the illuminated entry system.

Common Symptoms

• No Interior Lights on Entry/Exit: The most prominent symptom is the failure of the interior lights to illuminate when the doors are opened or when the vehicle is unlocked.
• Interior Lights Stay On or Flicker: Less common but possible, if the relay is intermittently shorted or stuck closed, causing lights to remain on or flicker erratically.
• Battery Drain (if relay stuck closed): In cases where the relay’s contacts are stuck in the closed position due to an internal fault, the illuminated entry lights may remain on continuously, leading to a parasitic battery drain.
• Other Courtesy Lighting Malfunctions: Depending on the specific vehicle’s wiring, other courtesy lights controlled by the same relay or circuit may also fail.

What Causes the Code B1371?

• Faulty Illuminated Entry Relay: The relay itself may have an internal short, open coil, or seized contacts, preventing proper operation.
• Damaged Wiring or Connectors: An open circuit, short to ground, or short to voltage in the wiring between the BCM and the relay, or in the relay’s power or ground supply circuits. This can include corroded, frayed, or disconnected wires and terminals.
• Blown Fuse: A fuse protecting the illuminated entry relay’s power supply or its controlled circuit may have blown, indicating an underlying short circuit or component failure.
• Corrosion at Relay Socket Terminals: Oxidation or corrosion on the relay pins or within the relay socket can create high resistance, preventing adequate current flow.
• Faulty Body Control Module (BCM): While less common, an internal failure within the BCM’s driver circuit for the illuminated entry relay can cause this code.

How to Diagnose and Troubleshoot

A systematic diagnostic approach is crucial for B1371. You will need an OBD-II scanner capable of accessing BCM codes, a digital multimeter (DMM), and potentially a wiring diagram for the specific vehicle.

1. Verify Code and Gather Information:
   • Connect an OBD-II scanner and confirm B1371. Check for any related BCM or other body electrical codes.
   • Note environmental conditions or events that preceded the fault.
2. Visual Inspection:
   • Locate the illuminated entry relay (consult a service manual for its exact location, often in an underhood or interior fuse/relay box).
   • Inspect the relay, its socket, and surrounding wiring for signs of damage, corrosion, melting, or loose connections.
   • Check the fuse(s) associated with the illuminated entry system and the relay’s power supply. Use a DMM to test continuity across the fuse, do not rely on visual inspection alone.
3. Relay Testing:
   • Remove the illuminated entry relay.
   • Using a DMM, measure the resistance across the relay’s coil terminals. Refer to service specifications, but typically, a good relay coil will show a resistance value (e.g., 50-150 ohms). An open circuit (OL or infinite resistance) indicates a faulty coil.
   • Apply battery voltage to the relay’s coil terminals (observe polarity if applicable) and listen for an audible click. Test continuity across the load (switch) terminals to confirm they close when energized.
   • If possible, swap the suspected relay with a known good, identical relay from a non-critical circuit (e.g., horn relay, if applicable) to see if the symptom changes or the code clears.
4. Circuit Integrity Testing (Relay Socket):
   • With the relay removed, use a DMM to test the relay socket terminals:
   • Power to Coil (86): Identify the relay coil’s power supply terminal (often 86). Using the DMM, measure voltage between this terminal and a known good chassis ground. It should show battery voltage when the BCM commands the relay ON (e.g., door open, remote unlock).
   • Ground to Coil (85): Identify the relay coil’s ground control terminal (often 85). The BCM typically provides a pulsed ground or direct ground to activate the relay. Measure resistance between this terminal and chassis ground. With the BCM commanding the relay OFF, it should show high resistance; when commanded ON, it should show near 0 ohms (good ground).
   • Power to Load (30): Identify the constant battery power supply terminal for the load (often 30). This should have constant battery voltage.
   • Output to Load (87): Identify the switched output terminal to the lights (often 87). This should show battery voltage when the relay is commanded ON and installed.
   • If any of these values are incorrect, trace the wiring back to the BCM, fuse box, or ground point, checking for opens, shorts, or high resistance. Pay close attention to splices and connectors.
5. BCM Output Test:
   • If all wiring and the relay test good, the issue might lie within the BCM. Access the BCM connector for the illuminated entry relay control circuit.
   • Using the DMM, measure the voltage or ground signal output from the BCM directly at the BCM connector pin when the system should be activated. Compare to service manual specifications. If the BCM is not providing the correct signal, it may be faulty.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

• Replace the Illuminated Entry Relay: If the relay tests faulty (open coil, stuck contacts, excessive resistance), replacement with an OEM or high-quality aftermarket equivalent is the primary solution. Ensure the replacement relay has the correct specifications (voltage, amperage, pin configuration).
• Repair or Replace Damaged Wiring/Connectors: For any identified open circuits, shorts, or high resistance in the wiring, repair the affected section using appropriate automotive wiring repair techniques (solder and heat shrink, not just electrical tape) or replace the entire harness section if damage is extensive. Clean any corroded connector pins and apply dielectric grease.
• Replace Blown Fuse: If a blown fuse is found, replace it with a fuse of the correct amperage. Importantly, investigate the cause of the blown fuse, as it often indicates an underlying short in the circuit, potentially in the relay itself or in one of the connected lighting components. Simply replacing the fuse without addressing the root cause will likely lead to a repeated failure.
• Clean Relay Socket Terminals: If corrosion is present, carefully clean the relay socket terminals using a small brush and electrical contact cleaner. Ensure a tight fit for the relay pins.
• Replace Body Control Module (BCM): If, and only if, all other components (relay, wiring, fuses) have been thoroughly tested and confirmed to be in good working order, and the BCM is definitively diagnosed as not providing the correct control signal, then BCM replacement may be necessary. BCM replacement often requires programming or calibration by a dealership or a shop with specialized equipment.

Mechanic’s Tip: Always check for Technical Service Bulletins (TSBs) related to B1371 or illuminated entry system issues for the specific vehicle make and model. Sometimes, manufacturers issue known fixes or updated parts that can save significant diagnostic time.