How do I fix OBD fault code B1491?

Inspect and reseat the connector at the ignition switch for corrosion or loose pins.. Check for cuts, pinches, or damaged insulation along the sensor wiring.. Replace the sensor if connector and wiring are intact.

OBD Code B1491: Ignition Cylinder Sensor Circuit Failure

Q: What does OBD code B1491 mean?

Ignition Cylinder Sensor Circuit Failure. Your car's ignition cylinder sensor isn't sending the right signals to the engine computer, like a broken lock that can't tell if the key is in the right position. This prevents the engine from starting or running properly.

Q: What are the symptoms of fault code B1491?

Engine won't start or has difficulty starting, Intermittent stalling while driving, Dashboard warning light illuminated

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What This Actually Means

In plain language — no jargon

Your car's ignition cylinder sensor isn't sending the right signals to the engine computer, like a broken lock that can't tell if the key is in the right position. This prevents the engine from starting or running properly.

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Symptoms You May Notice

3 known symptoms for this code

Engine won't start or has difficulty starting

Intermittent stalling while driving

Dashboard warning light illuminated

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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the ignition cylinder sensor's voltage signal to verify the ignition switch position (off, accessory, run, start). The sensor should transition smoothly between discrete voltage levels as the key rotates. A circuit failure is detected when voltage stays out of range or fails to respond to key position changes.

Voltage & Parameter Thresholds

Parameter	Normal Range	Fault Condition
Sensor Voltage	0.5V - 4.5V (varies by position)	Below 0.2V or above 4.8V / no signal
Signal Response Time	Transitions within 200ms of key movement	No transition or delayed >500ms

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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex

1

Ignition cylinder sensor connector

Inspect and reseat the connector at the ignition switch for corrosion or loose pins.

2

Wiring harness to ignition cylinder

Check for cuts, pinches, or damaged insulation along the sensor wiring.

3

Ignition cylinder sensor

Replace the sensor if connector and wiring are intact.

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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code B1491 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 B1491

What happens after you fix the fault

Once the fault is repaired, B1491 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.