Cause
Faulty immobiliser
The security system that authorises the engine to run, faulty so that a legitimate key is refused or authorisation cannot be confirmed. It is deliberately built to fail closed — an immobiliser that let the engine run when unsure would not be one — so the outcome is always a non-starting vehicle rather than a degraded one.
The commonest faults are at the ends, not in the module. A transponder in the key that has failed, an antenna ring around the ignition barrel that has broken or come unplugged, or a key that has physically worn are all far more likely than the control unit. Trying a second key is the fastest test available and separates key from vehicle in seconds.
Module replacement is the case that needs care. These components are cryptographically paired, so a replacement engine or body module must be programmed to the vehicle before it will authorise anything — and on many manufacturers that requires dealer-level access and proof of ownership. Establish that a module can be paired before it is bought, since a used one from another vehicle may never authorise on this one and is then worthless.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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