This 1 hour lesson is worth £150

Normally in my day job, I capture bits of knowledge, for myself, and others to save us time or money.  I'm not in work now and so I need to continue adding to a wiki somewhere else instead any big thing I learned, for others. Yesterday it was a week long being without a car. The keyfob broke and the car was immobilised.

1. The immobiliser talks to the ECU and tells it if it may run the fuel injectors, the immobiliser on my 1300 Suzuki Swift is thus really completely separate from the ECU and car, but if the unit is missing or bypassed, the engine just will not run. It will crank over but will not run.

2. The immobiliser light itself stays on, until the ECU is running, so it's not an indication of the key being good. Which is an annoying indicator lamp design, because you have to actually crank the engine over, to find out if the ECU and immobilizer are talking to each over the canbus (I don't have an OBD). And my battery is a pretty old battery.

So when I grabbed the spare key, to save the battery I did not try crank it, as the battery was low, I tried all the reset tricks, I pulled the battery off for 2 minutes. I locked and unlocked the car, closed all doors... but If I had known what I know now, I would have found the tiny transponder.

It's the same size as the one you put into your dog or cat, mine was black and rectangular, not glass.

It talks to a pickup coil or reader antenna around the ignition lock barrel. The chip needs no battery because it is actually briefly powered up by the NFC field for long enough to send back the correct challenge-response code.

Also, when undoing the battery, undo the ground terminal not the +, this was a bit counter intuitive to my mind, but that's probably good advice from a professional. The fact that I had not known what the small NFC chip looked like or where it was was not helpful.

The immobiliser is pretty dumb, it unlocks, and stays unlocked for a while once the key is present, meaning you can use the correct key to start the car, then stop the engine and use another key with no chip in it at all, to start and to continue using the car. Probably a safety timer margin to cover for RF interference causing failures to unlock.

Anyway, a lesson that cost £150, yours today at the special price of totally free.