How transponder keys work — and why a cut key won't start the car

A transponder key is two devices that happen to share a plastic shell. The metal blade is a mechanical key and does mechanical things — doors, boot, ignition barrel. The chip moulded into the head is a radio device with no battery, and it is the reason a perfectly cut blade turns the ignition and produces nothing.

How transponder keys talk to the immobiliser

Every start is a short conversation:

1. Turning the key or pressing start energises an antenna coil around the ignition or in the reader.
2. That field induces enough current in the chip to wake it. This is why it needs no battery — it borrows power from the car for the fraction of a second it is awake.
3. The immobiliser sends a challenge: a number, different every time.
4. The chip performs a calculation using a secret it holds and returns the answer.
5. The immobiliser runs the same calculation. Match, and fuel and ignition are released. No match, and the engine cranks and dies, or does not crank at all.

The important detail is that the secret itself never travels. Only the answer does, and because the challenge changes on every attempt, recording one exchange and replaying it is useless. That single design choice is why vehicle theft by key duplication collapsed after immobilisers became standard equipment.

The three parts people conflate

• The blade — mechanical, cut to a bitting, opens doors and turns the barrel.
• The transponder — passive, no battery, answers the immobiliser. Without it: no engine.
• The remote — battery-powered, transmits lock and unlock on a rolling code. Without it: everything still works, you just use the blade.

They fail independently, and knowing which has failed saves an entire callout. A key that locks the doors but will not start the car has a transponder or immobiliser problem. A key that starts the car but will not lock it remotely has a flat remote battery — the walkthrough is in flat battery or dead fob.

Why a "spare" from an online listing usually fails

A blank with the right blade profile and a compatible chip is only half a key. The chip still has to be written into your specific vehicle's immobiliser, and that is the part that needs equipment, access and the vehicle present. An uncut, unprogrammed blank is a component, not a key — a perfectly reasonable thing to buy if you know a locksmith is cutting and pairing it, and a waste of money if you expected it to work out of the envelope.

When the handshake starts failing intermittently

An engine that occasionally refuses to start and then starts fine on the third try is worth taking seriously before it becomes permanent. The usual causes are a cracked chip inside a key that has been dropped a lot, a failing antenna ring around the ignition barrel, or a worn barrel that no longer aligns the key where the reader expects it. A key that only works when held at a particular angle has already told you which one it is.

What this means practically

Two things. First, cutting a car key and programming a car key are separate jobs with separate equipment, and any quote that covers only one of them is not a quote for a working key. Second, if you have exactly one key, you are one loss away from an all-keys-lost job, which is the expensive version of the same visit.