P1621: Control Module Long Term Memory Performance/ Immobilizer Code Words Do Not Match

P1621 is a manufacturer-specific powertrain code that covers two related conditions: a performance fault in the control module's long-term memory, or a mismatch between the immobiliser code words stored in separate modules. In the memory performance context, it means the module has detected that its non-volatile storage is not reading back correctly, which can affect learned calibrations and adaptive values. In the immobiliser context, it means the code exchanged between the ECU and the immobiliser module does not match what is expected, which can prevent the engine from starting.

For the memory fault interpretation, the cause is typically an ageing or damaged control module where the internal memory has begun to fail. For the immobiliser mismatch, the fault often follows a module replacement where the new unit has not been correctly programmed to match the rest of the vehicle's security system. A flat or disconnected battery will not normally cause this code, but incorrect module replacement or a failed programming procedure will.

P1621 is medium severity and should not be ignored, particularly if the vehicle is showing starting difficulties or erratic behaviour linked to fuel delivery or ignition management. Resolving a module mismatch requires reprogramming with dealer-level diagnostic equipment. A technician will need to confirm which interpretation of the code applies to your specific vehicle before any work is carried out.