What Does B2791 Mean?
This code indicates the Body Control Module (BCM) has detected a malfunction in the key unlock warning switch circuit within the immobilizer system. This switch monitors when the ignition key is inserted or removed, and communicates with the vehicle's anti-theft immobilizer system. The BCM has identified an electrical fault, mechanical failure, or communication error in this circuit that prevents proper monitoring of key position for security system functions.
Common Causes
45%
Faulty ignition cylinder key unlock warning switch or internal sensor degradation
30%
Damaged, corroded, or loose wiring harness connector at the key unlock switch
15%
Short circuit, open circuit, or high resistance in wiring between switch and BCM
10%
Body Control Module internal fault or software corruption
Diagnostic Steps
1
Step 1: Perform visual inspection of the ignition cylinder area and locate the key unlock warning switch. Check for physical damage, moisture intrusion, or corrosion at the switch connector. Inspect wiring harness for chafing, pinching, or rodent damage.
2
Step 2: Using a digital multimeter, check switch connector terminals for corrosion and measure voltage supply at the switch (typically 5V or 12V reference). Check continuity of ground circuit to BCM with ignition off.
3
Step 3: Test the key unlock warning switch operation by measuring resistance or voltage output while inserting and removing the key. Compare readings to manufacturer specifications (typically switch should open/close circuit with key movement).
4
Step 4: If switch tests good, perform voltage drop test on all wiring circuits between switch and BCM with key cycled. Check for opens (infinite resistance) or shorts to ground/power (less than 10k ohms to ground when disconnected).
5
Step 5: If all wiring and switch test within specifications, verify BCM power and ground circuits are intact. Check for BCM software updates or known TSBs related to immobilizer system false codes.
6
Step 6: After repair, clear codes, cycle key multiple times through all positions, and verify code does not return. Test immobilizer system function to ensure proper communication restored.
Estimated Repair Cost
$150 - $650
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a malfunction in the key unlock warning switch circuit within the immobilizer system. This switch monitors when the ignition key is inserted or removed, and communicates with the vehicle's anti-theft immobilizer system. The BCM has ident...
The most common cause of B2791 (Key Unlock Warning Switch Malfunction (Immobilizer System)) is: Faulty ignition cylinder key unlock warning switch or internal sensor degradation
Typical repair costs for B2791 range from $150 to $650, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is generally safe to drive as this affects security monitoring rather than critical driving functions. However, immobilizer system may not function properly, potentially causing no-start conditions or leaving vehicle vulnerable to theft, so repair should be completed promptly.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B2791 to identify the root cause.
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Quick Info
Category
Body
System
Vehicle Security/Immobilizer System
Difficulty
Type
Generic (SAE)
Recommended Tools
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