B1372

What Does Code B1372 Mean?

DTC B1372 signifies an “Illuminated Entry Relay Circuit Open”. This code indicates that the vehicle’s Body Control Module (BCM), or in some vehicles, the Generic Electronic Module (GEM) or Smart Junction Box (SJB), has detected an open circuit condition within the electrical pathway controlling the illuminated entry relay. The illuminated entry system is responsible for activating interior courtesy lights, map lights, possibly exterior puddle lights, and key ring illumination when the doors are unlocked or opened. The BCM monitors the electrical continuity and voltage levels within the relay’s control circuit and/or its power output circuit. When an open circuit is present, the BCM detects an abnormally high resistance or a complete lack of continuity, preventing the relay from energizing or failing to supply power to the associated lighting circuits. This fault prevents the BCM from effectively commanding the illuminated entry system to operate, impacting driver and passenger convenience and safety.

Common Symptoms

• No interior courtesy lights (dome, map, footwell) when doors are opened or unlocked.
• Lack of illuminated key ring when the door is opened (if equipped).
• Inoperative puddle lights or approach lights (if equipped) upon vehicle unlock or entry.
• The Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) is typically not illuminated for B-codes, but a warning message may appear on the instrument cluster.
• Other body electrical functions controlled by the same module may exhibit intermittent operation or complete failure.

What Causes the Code B1372?

• Faulty Illuminated Entry Relay: The relay itself may have an internal open circuit in its coil windings or damaged contacts, preventing it from switching on.
• Open Circuit in Wiring: A break, cut, chafing, or severe corrosion in the power supply wire, ground wire, or the control circuit wire leading to or from the illuminated entry relay.
• Blown Fuse: A fuse protecting the power feed to the illuminated entry relay or its associated lighting circuits has opened.
• Corroded or Damaged Connector: Loose, corroded, or bent pins within the electrical connector to the illuminated entry relay, the Body Control Module (BCM), or associated light fixtures.
• Faulty Body Control Module (BCM/GEM/SJB): Internal malfunction within the control module preventing it from supplying the correct control signal or monitoring the circuit feedback accurately. This is less common but possible.

How to Diagnose and Troubleshoot

Diagnosing B1372 requires a methodical approach, primarily utilizing a digital multimeter (DMM) and a vehicle wiring diagram.

1. Visual Inspection: Begin with a thorough visual inspection of the wiring harness and connectors associated with the interior lighting system, specifically around the BCM (or GEM/SJB) and any known locations of the illuminated entry relay. Look for signs of chafing, corrosion, cuts, or loose connections.
2. Locate the Relay and Fuses: Consult the vehicle’s service manual to precisely locate the illuminated entry relay and its associated fuses. There may be multiple fuses for different courtesy lighting circuits.
3. Fuse Verification: With the ignition off, use a DMM set to continuity or resistance mode to check all fuses related to the interior lighting and BCM. An open fuse will indicate infinite resistance or no continuity. If a fuse is blown, note its amperage and do not immediately replace it without further investigation, as a short circuit elsewhere may be the underlying cause.
4. Relay Testing:
   • Remove the illuminated entry relay from its socket.
   • Using the wiring diagram, identify the relay’s coil terminals (usually 85 and 86) and switch terminals (usually 30 and 87/87a).
   • Measure the resistance across the coil terminals (85 and 86). A healthy coil should show a specific resistance value (typically 50-150 ohms, consult specifications); infinite resistance indicates an internal open coil.
   • Apply 12V and ground to the coil terminals (85 and 86). You should hear an audible click as the relay energizes. Then, measure continuity between the switch terminals (30 and 87). Continuity should be present when energized, and open when de-energized (for a normally open relay).
   • Alternatively, swap the suspect relay with a known good, identical relay from another non-critical circuit if available for a quick functional check.
5. Circuit Voltage and Continuity Checks: With the relay removed and using the wiring diagram:
   • Power Supply: Turn the ignition ON (or apply power as per diagnostic steps in the service manual). Use the DMM to check for 12V at the relay socket terminal corresponding to the B+ (battery positive) feed (usually terminal 30).
   • Ground: Check for a good ground connection at the relay socket terminal for ground (usually terminal 85 or 86, depending on the BCM control strategy).
   • Control Signal: With the system commanded ON (e.g., using an advanced OBD-II scanner’s bi-directional control, or by opening a door), check for the control signal from the BCM at the appropriate relay coil terminal (85 or 86). This signal can be 12V or a ground, depending on the circuit design. A missing or incorrect voltage/ground indicates a BCM issue or an open in the control wire.
   • Output Circuit: Check for continuity from the relay’s output terminal (usually 87) in the socket to the first component in the illuminated entry circuit (e.g., a dome light). Infinite resistance indicates an open circuit downstream of the relay.
6. Advanced Scanner Diagnostics: If available, use an advanced OBD-II scanner capable of bi-directional controls to command the illuminated entry relay ON and OFF directly from the BCM. Monitor relevant live data streams to observe if the BCM is attempting to activate the circuit and if any feedback is registered.

Recommended Repairs and Solutions

Once the root cause of the B1372 code has been identified through systematic diagnosis, the appropriate repairs can be performed:

• Replace the Faulty Relay: If the illuminated entry relay tests as internally open or defective, replace it with a new OEM or equivalent quality aftermarket relay.
• Repair or Replace Wiring: If an open circuit is found in the wiring harness, perform a professional repair. This involves splicing the wire using proper soldering techniques, heat shrink tubing, and ensuring the repair is waterproof and robust. For extensive damage, replacement of the affected harness section may be necessary.
• Replace Blown Fuse: If a blown fuse was identified, replace it with a fuse of the exact same amperage rating. However, it is crucial to investigate why the fuse blew. An underlying short circuit could cause the new fuse to blow immediately. Thoroughly inspect all components and wiring downstream of the fuse for shorts.
• Clean or Repair Connectors: If corrosion or loose/bent pins are found in any connector, clean the terminals thoroughly using electrical contact cleaner and a small brush. For bent or damaged pins, attempt to carefully straighten them or replace the connector housing and terminals if irreparable. Ensure all connections are secure and fully seated.
• Replace the Body Control Module (BCM/GEM/SJB): This should be a last resort after all other possibilities have been meticulously ruled out. A new BCM often requires programming or flashing to the vehicle’s specific options and VIN, which usually requires specialized diagnostic tools and expertise.
• Post-Repair Verification: After performing any repairs, clear the DTC B1372 using an OBD-II scanner. Operate the illuminated entry system multiple times (unlock doors, open doors) to confirm proper function. Drive the vehicle and re-scan to ensure the code does not return, indicating a successful repair.