Suden error code 17978 and difficulty in starting

Suddenly my 1999 Passat 1.8T has difficulty in starting and, amoungst others, I am getting a fault code 17978 / P1570 = "Engine Start Blocked by Immobilizer" from my generic DTC / OBD handheld reader.
:1zhelp:
Any suggestions on what I can check, investiagte or fix would be much apprecated.

Thank you

Is your key caked in dirt/etc? Not sure if it makes a difference, but remove the car key from any bunch of keys it might be hanging off and use it on its own, see if that makes a difference.

Thank you Sergiup - My key is clean but I will check that I look after it in future.

Today, I saw another post which suggested that not fully opening the ignition key from is folded / retracted position, may cause a problem.

I saw a tip on another web site suggesting that I should check for any poor connections in the wires to the "immobiliser control unit".

Could anyone tell me where I might find the ICU as I cannot see it in EKTA, nor in the Haynes Manaul, nor on this site nor anywhere else.

Thank you|