B1555

What Does Code B1555 Mean?

DTC B1555 signifies a detected malfunction within the vehicle’s ignition switch Run/Start Circuit. This code is typically set by the Body Control Module (BCM) or, in some architectures, directly by the Powertrain Control Module (PCM) or Engine Control Module (ECM). The BCM/PCM monitors the voltage signal provided by the ignition switch when it is moved to the “RUN” and “START” positions. This circuit is critical as it supplies power or a control signal to various essential vehicle systems, including but not limited to, the engine control unit, fuel pump relay, ignition coils, injectors, and accessory circuits. When the module detects that the voltage on this specific input circuit deviates from the expected range (e.g., continuously low voltage, an open circuit, or an unexpected short to ground/voltage) while the ignition switch is commanded to “RUN” or “START,” it interprets this as a failure and sets code B1555. This indicates that the module is not receiving the correct electrical input to confirm the ignition’s operational state or to power downstream components.

Common Symptoms

• No Crank/No Start Condition: The engine will not turn over when the key is turned to the “START” position.
• Intermittent Starting: The vehicle may start sometimes but not others, or require multiple attempts.
• Engine Cranks but Does Not Start: The starter engages, but the engine fails to ignite due to lack of fuel or ignition spark.
• Engine Stalling: The engine may unexpectedly shut off while driving or idling.
• Electrical Accessories Malfunction: Components like the radio, power windows, wipers, or HVAC fan may not operate or operate intermittently.
• Security Light Illumination: The anti-theft system light may illuminate on the dashboard, potentially preventing engine start.
• Warning Lights On: Other dashboard warning indicators (e.g., Check Engine Light, Battery Light) may illuminate.

What Causes the Code B1555?

• Faulty Ignition Switch: Internal wear, corrosion, or mechanical failure of the electrical contacts within the ignition switch, leading to intermittent or complete loss of continuity in the Run/Start circuit.
• Open or Short Circuit in Wiring: Damaged, chafed, or broken wiring in the ignition Run/Start circuit between the ignition switch and the BCM/PCM, causing an open circuit, a short to ground, or a short to voltage.
• Blown Fuse: A blown fuse protecting the ignition Run/Start circuit or a related accessory circuit drawing power from it.
• Corroded or Loose Electrical Connectors: Poor electrical connection at the ignition switch, inline harness connectors, or at the BCM/PCM module due to corrosion, pin push-out, or inadequate terminal tension.
• Faulty Ignition Relay: If the ignition Run/Start circuit incorporates a dedicated relay, failure of this relay can cause the code.
• Failed Body Control Module (BCM) or Powertrain Control Module (PCM): While less common, an internal fault within the BCM or PCM that prevents it from correctly monitoring or interpreting the ignition switch input signal can set this code.

How to Diagnose and Troubleshoot

Diagnosing B1555 requires a methodical approach, focusing on the electrical integrity of the ignition Run/Start circuit. Always begin with basic checks and progress to more detailed component testing.

1. Verify Battery Voltage and Grounds: Ensure the battery is fully charged (minimum 12.4V static) and all main power and ground connections are clean and secure. A weak battery can cause various electrical anomalies.
2. Scan for Related DTCs: Connect an OBD-II scanner and confirm B1555. Check for any other related B-codes (Body) or P-codes (Powertrain) that might indicate a broader electrical issue or a module communication problem. Note any freeze frame data.
3. Visual Inspection:
   • Visually inspect the ignition switch assembly for obvious mechanical damage or looseness.
   • Examine the wiring harness emanating from the ignition switch, tracing it as far as possible for signs of chafing, cuts, or burning.
   • Inspect all relevant fuses in the underhood fuse box and interior fuse panel. Use a digital multimeter (DMM) to check for continuity across each fuse. Do not rely solely on visual inspection. Pay particular attention to fuses labeled IGN, BCM, PCM, or ACC.
   • Check for corrosion or damage at the ignition switch connector and any inline connectors in the circuit path.
4. Ignition Switch Output Test (Using DMM):
   • Obtain a vehicle-specific wiring diagram to identify the correct wires for the ignition Run/Start circuit at the ignition switch connector.
   • With the connector still attached (back-probing) or disconnected (depending on the test), use a DMM to measure voltage on the specified wires with the key in “ACC,” “RUN,” and “START” positions.
   • In “RUN” and “START,” the designated circuit wire(s) should show close to battery voltage (e.g., 12V-12.6V). If voltage is missing or significantly low only in these positions, the ignition switch itself is likely faulty.
5. Circuit Integrity Test (From Ignition Switch to BCM/PCM):
   • If the ignition switch output is correct, the issue lies in the wiring or the receiving module.
   • Locate the BCM/PCM connector(s) and identify the specific input pin for the ignition Run/Start circuit using the wiring diagram.
   • Disconnect the negative battery terminal. Disconnect both the ignition switch connector and the BCM/PCM connector.
   • Perform a continuity test on the wire(s) corresponding to the Run/Start circuit between the ignition switch connector and the BCM/PCM connector. Expect very low resistance (typically < 5 ohms). High resistance indicates an open circuit or damaged wire.
   • Perform a short to ground test by placing one DMM lead on the wire and the other on a known good chassis ground. Expect infinite resistance (OL). Any measurable resistance indicates a short to ground.
   • Perform a short to voltage test by re-connecting the battery (with connectors still disconnected at ignition switch and BCM/PCM) and checking for voltage on the wire. Expect 0V. Any voltage indicates a short to power.
6. BCM/PCM Input Voltage Test:
   • Reconnect the ignition switch, but keep the BCM/PCM connector back-probed (or use a breakout box if available).
   • With the key in “RUN” and “START” positions, measure the voltage on the identified input pin at the BCM/PCM.
   • If battery voltage is present at the ignition switch output but absent or significantly low at the BCM/PCM input, the wiring between them is faulty.
   • If battery voltage is present at the BCM/PCM input, but the code B1555 persists, suspect an internal fault within the BCM/PCM itself.

Recommended Repairs and Solutions

Once the root cause of B1555 has been identified through systematic diagnosis, the following repairs are commonly performed:

• Ignition Switch Replacement: This is the most frequent repair for B1555. Replace the faulty ignition switch with an OEM or high-quality aftermarket equivalent. On some vehicles, the ignition switch may be integral to the steering column lock mechanism or incorporate transponder antenna for immobilizer systems, requiring specialized tools for removal and potentially key programming.
• Wiring Repair or Replacement: If an open circuit, short to ground, or short to voltage is found in the wiring, the damaged section of the harness must be repaired or replaced. Utilize appropriate soldering techniques with heat-shrink tubing for durable repairs, or replace the entire sub-harness if damage is extensive. Always ensure repairs are moisture-proof and securely routed.
• Fuse Replacement and Root Cause Investigation: If a blown fuse is discovered, replace it with a fuse of the exact same amperage rating. Subsequently, identify and rectify the underlying cause of the fuse blowing (e.g., a short in an accessory circuit powered by the same fuse). Simply replacing a fuse without addressing the short will lead to immediate re-failure.
• Connector Service/Replacement: Clean corroded terminals using electrical contact cleaner and a small brush. Repair bent or damaged pins. If the connector housing or terminals are severely damaged or corroded, replace the connector using appropriate terminal crimping tools. Ensure proper pin retention.
• Ignition Relay Replacement: If the diagnosis points to a faulty ignition relay, replace it with a new, tested unit. Confirm the relay is receiving proper control signals and power before condemning it.
• BCM/PCM Replacement and Programming: This should be a last resort, only after ruling out all other external circuit issues. If a module replacement is necessary, it will typically require specialized diagnostic equipment for programming, calibration, and initialization to the vehicle’s specific options and immobilizer system. This is often a dealer-level repair.

Mechanic’s Tip: Always clear the DTCs after completing any repair and perform a comprehensive test drive or multiple key cycles to confirm the code does not return. Pay close attention to vehicle behavior during starting and driving conditions that previously triggered symptoms. For intermittent issues, applying light pressure or wiggling connectors/wires in the suspected circuit path during live data monitoring can sometimes help pinpoint a loose connection or chafed wire.