Hello, I have a 2006 Octavia , 1.9tdi L&K , car has been brilliant until now, I started getting the odd low oil pressure on the dash bleep now and again, then the odd morning where it would start then immediately stop and the immobiliser key sign on the dash. 
All electronic related.
Now it just starts and immediately stops. 
I sent the clocks off to get them checked out, clocks come back but same problem is still happening. 
I have removed the ecu from the front scuttle for testing. 
No corrosion on the ecu pins at all 

All fuses are fine. 
Can anyone advise if I’m on correct procedure or have I missed something else? 

Cheers 

Had the same in a Seat 1.9tdi. The imobilizer came on after using the spare fob. The transponder chip  in the spare key had failed at some point before I bought the car and didn't realise. My problem was the imobilser stuck on and the other key wouldn't work either.  Eventualy after leaving the car battery disconnected for a couple of weeks it started working again after I  touched  battery cables together when disconnected. 

You could try another fob if you have one. Make sure you use a fob with the buttons as not sure if the other small key without the buttons has a transponder in it? 

It could also be the transponder reading coil in the ignition switch that has failed and isn't reading the key.  

The only other option would be to disable the imobilser in the ECU but you would need VCDS and know how to do it. Not the info thats generally put up on line. 

If the transponder chip has failed then a car locksmith may be able to retrieve the code and reprogram it for you.

Also make sure the car battery is good and fully charged.

Alasdair

It did have at least half dozen error codes come up , but I’ve just I’ve just gone back out to it and reattached the ecu, bloody thing started straight away and ran fine 

I plugged in the diagnostic and it’s showing no codes present 

I don’t have a spare key for it 

Edited 16 February, 20251 yr by Sam19700

Its typical when that happens. My Seat imobilser was stuck on so ended up buying a Skoda octavia to keep me going. Its a very long story!!. The Seat was sitting with battery disconected over December a few years back.  Start of Jan I thouht I try again charged baattery and perfect. No codes nothing. Started first time. Wonder if the ECU had a blip and needed to be reset, probably never know but Ive never used the spare key again and its never happened again. If you disconnected the ECU and reconnected it it may haave reset it. Hope it doesnt give you any more bother.

Alasdair