B1557

Ignition Run/Start Circuit Short To Battery

Body Chassis/Safety Ignition Control 🟢 Low — Fix at next service ✅ Safe to Drive
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What This Actually Means

In plain language — no jargon

The ignition switch circuit is receiving too much voltage, like a wire touching the positive battery terminal when it shouldn't. This tricks the ECU into thinking the ignition is always in the run or start position.

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Symptoms You May Notice

3 known symptoms for this code
Difficulty starting or no-start condition
Dashboard warning lights staying on continuously
Electrical components activating unexpectedly
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the voltage signal from the ignition switch circuit to determine its position (off, run, start). It expects specific voltage levels for each position. When voltage exceeds the normal range for the run/start circuit, the ECU detects a short-to-battery fault condition.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
Ignition Run/Start Signal Voltage 4.5-5.5V (logic high) or 0-0.5V (logic low) >5.5V or shorted directly to battery voltage (12V+)
Circuit Resistance 10-100 ohms in normal operation <1 ohm indicating direct short path
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Wire harness connector at ignition switch
Inspect and reseat all connectors at the ignition switch assembly to ensure no loose pins are touching the battery voltage line.
2
Ignition switch assembly
Replace the ignition switch if internal contacts are corroded or welded together, causing constant battery voltage output.
3
Wiring harness and protective sheathing
Check for damaged insulation along the ignition run/start circuit wires and replace any sections with exposed conductors.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code B1557 is a low-severity fault. Your vehicle is generally safe to drive to a workshop for diagnosis. However, do not ignore it indefinitely — low-severity codes often indicate developing problems that become expensive if neglected. Book a diagnostic appointment within 2–4 weeks. If you notice any additional symptoms (rough running, power loss, unusual smells), treat it as higher priority.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code B1557

What happens after you fix the fault

Once the fault is repaired, B1557 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.