B1556

Code

Other B — Body

Ignition Run/Start Circuit Open

Brand: Other

Type: B — Body

Views: UK: 29 EN: 39 RU: 33

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Page language: EN

Causes

Open or high-resistance wiring between ignition switch and control module
Faulty or worn ignition switch (mechanical/electrical contacts)
Blown fuse or failed start/run relay
Corroded, loose, or damaged connector at switch, relay, or control module
Faulty body control module (BCM) / ignition control module
Aftermarket alarm/immobilizer or remote start incorrectly wired

Symptoms

No crank or no start when key is turned to START
Intermittent starting or starts only after moving the key
Accessories or dash indicators not powered in RUN position
Starter does not engage despite turn-to-start action
Security/immobilizer warnings or unexpected disablement of starting

What to check

Scan vehicle for B1556 and any related codes; note freeze-frame data and whether code is current or historic
Visually inspect fuses and relays for the ignition/run circuits; replace if blown or suspect
Inspect connectors at the ignition switch, starter relay, and BCM for corrosion, bent pins, or poor seating
Back-probe ignition switch RUN and START terminals while operating key and measure voltage
Perform continuity checks between ignition switch terminals and the control module input with key off and key in RUN/START
Check for evidence of aftermarket alarm/remote-start installations and verify correct wiring

Signal parameters

Ignition RUN/START input: approximately battery voltage (~11–14 V) when key in RUN or during START cranking
Ignition RUN/START input: near 0 V when key is OFF
Continuity between switch terminal and module input should be low resistance (
Starter relay control expected to receive a control voltage during START (momentary) and be grounded or driven per vehicle design

Diagnostic algorithm

Confirm code and attempt to reproduce: record whether code sets with key in RUN, START, or both and whether code is permanent or intermittent.
Inspect fuses and relays related to ignition/run/start circuits; replace any suspect items and re-check.
Visually examine the ignition switch connector, steering column wiring, and BCM connector for corrosion, damage, or disconnected pins.
Back-probe the ignition switch RUN and START terminals. With a DVOM, verify battery voltage appears at the RUN terminal with key in RUN and at the START terminal only while cranking. If voltage is absent at the switch terminals, suspect the switch or upstream power feed.
Back-probe the corresponding input at the BCM or ignition control module while operating the key. If voltage is present at the switch but not at the module, perform a continuity/ resistance check of the wiring harness between the two points and inspect connectors in between.
Wiggle-test connectors and harness while monitoring signal to find intermittent opens. If intermittent, repair/replace affected wiring or connector.
If wiring and switch check OK, test or substitute a known-good ignition switch and/or start/run relay (as applicable) per manufacturer procedure.
If switch, wiring, fuses, and relays are good but input still open at the module, consider module input circuit failure; confirm with manufacturer diagnostics before replacing module.
After repairs, clear codes and perform multiple key cycles and start attempts to confirm code does not return. Verify related systems (immobilizer, remote start) function correctly.

Likely causes

Broken/disconnected wire in the run or start signal circuit
Damaged ignition switch contact failing to make connection in RUN or START positions
Blown fuse feeding the ignition run/start input
Connector corrosion or bent pins at the ignition switch or BCM
Faulty BCM/input circuit open internally
Interference from aftermarket security or remote-start devices

Fault status

⚠️ Status

Ignition Run/Start Circuit Open — the control module detects no expected RUN or START signal (open circuit) on the ignition input. This may prevent the vehicle from recognizing key position and can inhibit starting.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours