Engine cranks but doesn't start. Dashboard immobiliser light flashing. Key fob recognised then rejected. All classic symptoms of an immobiliser fault — and almost always fixable mobile, without a dealer.
What an immobiliser actually does
The immobiliser ECU stores a list of approved transponder IDs. When you turn the key or press start, it reads the chip in your key and only allows the engine to fire if the ID matches. If anything in that chain breaks, the car cranks but won't start.
The 3 most common causes
1) Failed transponder coil around the ignition barrel — extremely common on Vauxhall, Ford and older Audi. 2) Damaged immobiliser module (CAS / EWS / PATS) — common on BMW E-series. 3) Key out of sync with the immobiliser, especially after a flat battery or jump start.
How we fix it on-site
Mobile diagnostics first to confirm the exact fault, then transponder coil replacement, key reprogramming or full immobiliser EEPROM repair as needed — all from our van at your driveway or roadside.
When it's NOT the immobiliser
If the engine cranks slowly or not at all, the problem is more likely your battery, starter motor or fuel system. A proper diagnostic separates the two in 15 minutes.
Related services on StageMyCar: auto locksmith in Harrow and key & immobiliser programming.