B1393

What Does Code B1393 Mean?

DTC B1393 signifies an Open Circuit condition detected within the Power Door Memory Lock Relay Circuit. This code is typically set by the Body Control Module (BCM) or a dedicated Door Control Module (DCM) responsible for supervising the vehicle’s body electrical functions, including power door locks, particularly those integrated with memory functions (e.g., driver 1, driver 2 settings). The relay in question is crucial for supplying electrical current to the door lock actuators, allowing them to engage or disengage the locking mechanism, often in conjunction with remote keyless entry (RKE) or integrated driver memory systems.

The control module monitors the electrical continuity and voltage integrity of this specific circuit. When the module attempts to energize the memory lock relay, it expects to see a specific voltage drop or current flow indicative of a closed circuit. If, instead, it detects an abnormally high resistance or a complete lack of continuity—signifying a break in the electrical path (an “open circuit”)—it interprets this as a fault. This diagnostic information is then communicated via the vehicle’s data bus (e.g., CAN bus) to the Powertrain Control Module (PCM), where it is stored as B1393 and illuminates the Malfunction Indicator Lamp (MIL) or an equivalent service indicator if applicable, though for body codes, a dedicated warning light or message might be more common.

Common Symptoms

• Inoperative Power Door Lock: The most direct symptom is that the power door lock for the affected door (often the driver’s front door) will not operate via the interior switch or remote keyless entry.
• Memory Lock Functions Disabled: Any integrated memory features tied to the door lock (e.g., automatic locking/unlocking linked to driver profiles) will fail to function.
• Remote Keyless Entry Malfunction: The remote fob may not be able to lock or unlock the specific door associated with the fault, though other doors might function correctly depending on the vehicle’s wiring architecture.
• Absence of Relay Click: No audible click from the relay can be heard when attempting to actuate the door lock, indicating the relay is not energizing.
• Warning Messages: Some vehicles may display a “Door Lock Malfunction” or similar warning message on the instrument cluster.

What Causes the Code B1393?

• Faulty Power Door Memory Lock Relay: The relay itself may have an internal open circuit in its coil or contacts, preventing it from switching power to the lock actuator.
• Open Circuit in Wiring Harness: A break, fray, or corroded section of wire within the circuit leading to or from the relay, or within the door jamb boot where wires are prone to fatigue from repeated door opening/closing.
• Blown Fuse: A fuse protecting the relay coil power supply or the power feed to the relay’s load contacts may have blown, creating an open circuit.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the relay socket, the door control module connector, or any inline connectors within the circuit due to corrosion, bent pins, or inadequate terminal tension.
• Faulty Door Control Module (DCM) or Body Control Module (BCM): Less common, but an internal fault within the controlling module (DCM/BCM) could prevent it from supplying the necessary ground or power signal to energize the relay coil, or from correctly monitoring the circuit.

How to Diagnose and Troubleshoot

Diagnosis of B1393 requires a systematic approach, starting with basic inspections and progressing to electrical circuit testing using a digital multimeter (DMM) and an advanced OBD-II scan tool.

1. Retrieve and Record DTCs: Use an OBD-II scan tool to confirm B1393 and check for any related or accompanying body electrical codes. Note any freeze frame data that might provide context regarding the conditions under which the fault occurred.
2. Visual Inspection:
   • Locate the Power Door Memory Lock Relay using a vehicle-specific wiring diagram. Inspect the relay and its socket for signs of corrosion, burning, or physical damage.
   • Inspect all fuses related to door locks, body control modules, and the memory system. Replace any blown fuses, but be prepared to investigate the underlying cause (e.g., a short circuit).
   • Examine the wiring harness passing through the door jamb boot for the affected door. Look for chafed, cut, or pinched wires, especially where the harness flexes.
   • Inspect all relevant electrical connectors (at the BCM/DCM, relay, door lock actuator) for corrosion, loose terminals, or bent pins.
3. Relay Testing (Digital Multimeter – DMM):
   • Coil Resistance Test: Remove the relay. Identify the coil terminals using the wiring diagram. Measure the resistance across these terminals. An “OL” (open loop) reading indicates an open coil, meaning a faulty relay. Compare to manufacturer specifications (typically 50-200 ohms).
   • Power and Ground Supply to Relay Coil: With the relay removed, use the DMM to check for battery voltage (B+) at one coil terminal and a solid ground at the other when the door lock system is commanded to operate (e.g., pressing the lock button). If either is missing, the problem lies in the BCM/DCM’s output or the wiring leading to the relay.
   • Power Supply to Relay Contacts: Identify the constant B+ input terminal for the relay’s contacts (powering the load). Ensure full battery voltage is present at all times.
4. Circuit Continuity Testing:
   • With the battery disconnected and relevant components unplugged, use the DMM’s continuity function to test for an open circuit in the wires.
   • Test continuity from the relay socket to the BCM/DCM and from the relay socket to the door lock actuator. Wiggle the harness during testing to identify intermittent opens.
5. Scan Tool Active Test: If available, use an advanced scan tool to command the door lock relay on and off. While commanding, use the DMM to verify that the BCM/DCM is correctly sending power/ground signals to the relay coil.

Recommended Repairs and Solutions

Once the root cause has been isolated through thorough diagnosis, the following repairs are typically applicable:

• Replace Faulty Relay: If the relay itself is found to have an internal open circuit, replace it with a new OEM or equivalent quality relay. Mechanic’s Tip: Always verify the new relay’s specifications match the original.
• Repair or Replace Damaged Wiring: If the visual inspection or continuity tests reveal damaged wiring, carefully repair the affected section. For critical circuits, soldering and proper heat-shrink insulation are recommended. For extensive damage, replacing the relevant section of the wiring harness may be necessary. Ensure proper strain relief for wires passing through door jamb boots.
• Replace Blown Fuse and Investigate Cause: If a blown fuse is found, replace it. However, a fuse rarely blows without an underlying cause. Thoroughly check the circuit protected by that fuse for any short circuits to ground that could cause repeated fuse failures.
• Clean and Secure Connectors: For corroded or loose connectors, disconnect them, clean the terminals using electrical contact cleaner and a small brush, then apply dielectric grease before reassembly to prevent future corrosion. Ensure terminals are properly seated and secured.
• Replace Door Control Module (DCM) or Body Control Module (BCM): If all wiring, fuses, and the relay itself test good, and the module’s output signals to the relay coil are confirmed to be absent or incorrect, the DCM or BCM may be internally faulty. Mechanic’s Tip: Module replacement often requires specialized programming or calibration with a factory-level scan tool. Always consult the factory service manual for specific procedures.

After any repair, clear the DTCs with the scan tool and perform multiple functional tests of the door lock memory system to ensure the fault does not recur. Drive the vehicle and re-check for the code to confirm a successful repair.