Peugeot 206 Engine immobilizer fault?

I have a Peugeot 206 sport 1.4 litre petrol 2006 model. Today I try to start my car and it shows up Engine Immobilizer fault. The car just turns when try to start up but will not fire up. Now it says Economy mode active. I have tried re syncing fobs - they are all OK - I've tried all my 3 keys. ( all lights etc all work ok just cant power the car up with key because of this stupid error) I am going to disconnect the battery for half hour and see what happens after that . - But any suggestions on the fix for this problem.