What Does B1491 Mean?
This code indicates that the Body Control Module (BCM) has detected an electrical fault in the ignition cylinder sensor circuit. The ignition cylinder sensor monitors the position of the key in the ignition lock cylinder and communicates this status to the BCM for security and accessory power management. The BCM has identified either an open circuit, short circuit, excessive resistance, or loss of signal from this sensor.
Common Causes
40%
Faulty ignition cylinder sensor or worn ignition lock cylinder causing intermittent or no signal output
30%
Damaged, corroded, or loose wiring harness connector at the ignition cylinder sensor
20%
Broken or chafed wiring between the ignition cylinder sensor and BCM causing open or short circuit
10%
Failed Body Control Module unable to properly read or process ignition cylinder sensor signals
Diagnostic Steps
1
Step 1: Perform a visual inspection of the ignition cylinder sensor connector and wiring harness for obvious damage, corrosion, loose pins, or disconnected connectors. Check for bent pins or moisture intrusion in the connector.
2
Step 2: Using a digital multimeter, backprobe the ignition cylinder sensor connector and verify reference voltage (typically 5V or 12V depending on manufacturer) at the sensor power supply pin with ignition on. Compare readings to manufacturer specifications.
3
Step 3: Check for proper ground at the ignition cylinder sensor ground circuit using a multimeter. Verify less than 0.5 ohms resistance to known good chassis ground. If ground circuit is open or high resistance, trace wiring for breaks or corrosion.
4
Step 4: With the sensor connector disconnected, measure resistance across the sensor terminals while rotating the key through all ignition positions (OFF, ACC, ON, START). The resistance should change predictably with key position. Compare values to manufacturer specifications.
5
Step 5: Inspect the steering column and harness routing for any signs of wire chafing, pinching, or damage from previous repairs. Pay special attention to areas where the harness flexes during steering column adjustment.
6
Step 6: If all wiring and sensor tests pass, verify BCM communication and power/ground circuits. Clear codes, cycle the ignition multiple times through all positions, and retest. If code returns immediately with known-good sensor and wiring, suspect BCM failure.
Estimated Repair Cost
$150 - $650
Parts + labor, varies by vehicle and location
This code indicates that the Body Control Module (BCM) has detected an electrical fault in the ignition cylinder sensor circuit. The ignition cylinder sensor monitors the position of the key in the ignition lock cylinder and communicates this status to the BCM for security and accessory power manage...
The most common cause of B1491 (Ignition Cylinder Sensor Circuit Failure) is: Faulty ignition cylinder sensor or worn ignition lock cylinder causing intermittent or no signal output
Typical repair costs for B1491 range from $150 to $650, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle may still be drivable but could experience issues with accessory power control, anti-theft system activation, or no-start conditions. Repair within a few days as the vehicle may fail to start unexpectedly or become disabled by security system malfunction.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1491 to identify the root cause.
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Quick Info
Category
Body
System
Vehicle Security and Ignition System
Difficulty
Type
Manufacturer
Recommended Tools
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