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Ignition Run/Start Circuit Open

Jump to the fix

Severity

Medium

Can I keep driving?

Usually drivable, but get it diagnosed without delay.

First step

Measure the voltage on the run/start input at the module with the key in RUN, then again while cranking; it must stay close to battery voltage in both

  • BBody
  • 1Manufacturer-specific
  • 5Body
  • 56Sequence number

What does it mean?

B1556 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Seat slide sensor system for fault 2 (malfunction of seat slide sensor) .

Symptoms

  • The affected module does not wake with the key on, so its lighting, wiper, lock or immobiliser functions are dead
  • Intermittent no-crank, or the engine cuts out while driving and restarts after a key cycle
  • Several unrelated body codes are stored at the same time, all from the same module
  • Dash warning lamps flicker or drop out when the key is turned to START
  • Airbag or occupant lamp illuminated where the code refers to the seat position sensor

Likely causes

  • Blown ignition fuse feeding the module, or a fuse holder with a burnt contact
  • Open or high resistance in the ignition run/start feed wire, often at a taped splice behind the dash
  • Worn ignition switch contacts that drop out in the RUN or START position
  • Loose or corroded module connector, or a terminal that has backed out of its housing
  • Faulty relay passing the run/start feed, or a corroded relay socket
  • Poor module ground that behaves exactly like an open supply
  • On applications where this code refers to the seat position system, an open or failed seat slide sensor under the seat

How to fix it

  1. Measure the voltage on the run/start input at the module with the key in RUN, then again while cranking; it must stay close to battery voltage in both
  2. Check the ignition fuse and its holder, and look for heat discolouration around the terminals
  3. Wiggle-test the ignition switch and the column loom while watching the voltage on a meter set to record minimum values
  4. Inspect the module connector for backed-out and spread terminals, and re-seat the connector fully
  5. Do not forget the ground side: measure the voltage drop from the module ground pin to the battery negative post under load
  6. Repair the open circuit, replace the relay if it drops voltage under load, and replace the ignition switch only after proving its contacts fail
  7. On the seat sensor variant, inspect the under-seat harness and test the slide sensor against the manufacturer values

Codes From the Same System

All Fault Codes

Related Similar Codes

This information follows the generic OBD-II definition and does not replace a vehicle-specific diagnosis. The severity rating is rule-based; an authorised diagnostic reading is required for a definitive answer.