B1555: Ignition Run/Start Circuit Fault
The B1555 code indicates a fault in the ignition Run/Start circuit, specifically in detecting the ignition switch position or the signal that tells the vehicle's control module the engine is in RUN or START mode. This circuit is responsible for informing the Body Control Module (BCM) or Powertrain Control Module (PCM) about the ignition state. The failure can cause no-start conditions, loss of communication between modules, or inoperative electrical functions in the ignition position.
1. Commonly Affected Makes
2. Most Common Causes
- Defective or worn ignition switch
- Open or short circuit in the RUN/START signal wiring
- Corroded or loose ignition switch connector
- Faulty ignition relay
- Failed Body Control Module (BCM)
- Damaged ignition circuit protection diode
- Weak battery or poor ground connections
3. Symptoms You Will Notice
- Engine cranks but won't start
- No crank or start
- Loss of communication between modules
- Electrical accessories inoperative in RUN position
- Instrument cluster warning lights may not illuminate
4. Step-by-Step Diagnosis
Check battery voltage and ground connections; repair if low or corroded.
Inspect ignition switch connector for corrosion, damage, or looseness; clean or replace as needed.
Test ignition switch continuity in OFF, RUN, and START positions; replace if faulty.
Check for open or short in the RUN/START signal wire between ignition switch and BCM/PCM.
Test ignition relay operation and replace if defective.
If all else passes, suspect BCM or PCM failure; perform module communication test.
5. Frequently Asked Questions
Is it safe to drive?▼
No, this is a high-severity fault. The vehicle may stall or fail to start, and loss of communication can affect safety systems. Have it diagnosed immediately.
How much does it cost to fix?▼
Repair costs range from $100 to $600, depending on the cause. A simple ignition switch replacement may be $100-$250, while a BCM replacement can exceed $500.