What Does B1556 Mean?
This body control module code indicates an open circuit has been detected in the ignition run/start circuit. The BCM has identified that proper voltage or continuity is not present in the ignition switch feed circuit that provides power to various vehicle systems during run and start positions. This prevents the BCM from detecting proper ignition switch position and can affect starting, accessory operation, and communication with other control modules.
Safety Warning
Vehicle may fail to start or could stall unexpectedly while driving if ignition circuit completely opens. Critical safety systems and engine operation may be affected. Do not drive until repaired.
Common Causes
40%
Failed ignition switch or worn ignition switch contacts causing intermittent or complete loss of circuit continuity
30%
Broken or corroded wiring between ignition switch and BCM, often in steering column harness or under-dash connections
20%
Blown fuse or fusible link in the ignition run/start power distribution circuit
10%
Faulty BCM internal circuit or BCM connector terminal corrosion preventing proper signal detection
Diagnostic Steps
1
Step 1: Perform visual inspection of ignition switch connector and BCM connectors for corrosion, pushed-out pins, or damage. Check all ignition-related fuses in both underhood and interior fuse boxes for continuity.
2
Step 2: Using a digital multimeter, backprobe the ignition switch connector with key in RUN position and verify battery voltage is present on the run/start output circuit (typically 12-14V). Compare to wiring diagram for vehicle-specific pin identification.
3
Step 3: With multimeter still connected, wiggle ignition switch and steering column harness while monitoring voltage to identify intermittent opens. Cycle key from OFF to RUN to START positions and verify proper voltage transitions.
4
Step 4: Measure voltage at BCM connector ignition input terminal with key in RUN position. If voltage is present at switch but not at BCM, trace and repair open in wiring harness between components.
5
Step 5: If voltage is present at BCM connector, check resistance of BCM ground circuits (should be less than 1 ohm). If grounds are good and voltage is present, suspect internal BCM fault.
6
Step 6: After repairs, clear codes and cycle ignition switch through all positions multiple times while monitoring live data to confirm BCM recognizes all ignition switch positions properly.
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
This body control module code indicates an open circuit has been detected in the ignition run/start circuit. The BCM has identified that proper voltage or continuity is not present in the ignition switch feed circuit that provides power to various vehicle systems during run and start positions. This...
The most common cause of B1556 (Ignition Run/Start Circuit Open) is: Failed ignition switch or worn ignition switch contacts causing intermittent or complete loss of circuit continuity
Typical repair costs for B1556 range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle may fail to start or could stall unexpectedly while driving if ignition circuit completely opens. Critical safety systems and engine operation may be affected. Do not drive until repaired.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1556 to identify the root cause.
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Quick Info
Category
Body
System
Body Control and Electrical System
Difficulty
Type
Manufacturer
Recommended Tools
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