Won’t start, and the immobiliser light stays on

If your dash is lighting up with that little key/car padlock icon and the engine won’t crank or fire, the immobiliser system isn’t letting your car start.

Why It Happens

Your immobiliser is basically an anti-theft handshake: key (or fob) sends a code → car’s ECU checks it → if the codes match, fuel and ignition are unlocked. If not, no start.

When the light stays on instead of going out after a second or two, the handshake failed.

Most Common Causes

• Key/Fob issue – flat fob battery, damaged transponder chip, or using the wrong key.
• Reader coil fault – the ring antenna around the ignition barrel (or the push-button sensor) isn’t seeing the key’s signal.
• Immobiliser control fault – wiring, ECU glitch, or module failure.
• Low car battery voltage – modern systems refuse to play ball if supply voltage dips.

Quick Checks You Can Try

1 Try your spare key/fob. If it works, you know the original is faulty.

4 Move the fob away from other keys/electronics. Interference can block the signal.

What a Mechanic Will Check

• Scan the system for fault codes (e.g. “Key Not Recognised” or “Immobiliser Communication Error”).
• Test the antenna coil at the ignition barrel for continuity and signal.
• Check immobiliser/ECU wiring and grounds.
• Verify if the immobiliser module itself has failed (rare, but expensive).

Ballpark Repair Costs

• New fob battery – $5–$15.
• Spare key/fob programming – $80–$250 depending on brand.
• Reader coil replacement – $120–$250 parts & labor.
• Immobiliser/ECU repair or replacement – $400–$900+ (dealership-level job).
• Car battery replacement – $120–$200.

When to Call It Quits

If the spare key doesn’t work and the immobiliser light stays on, it’s usually beyond driveway fixes. At this stage, you’ll need a scan tool and probably a locksmith or dealer-level repair.