If i replace the ECU with a used one from the same model car, will the car start and die a few seconds later until I get it reprogrammed? I bought a used ECU and swapped it out but when I took it to the shop they told me the ECU couldnt be the problem because it did the same thing the old ecu did (crank but no start). He said if it was the ECU the car would start and then die a few seconds later until he reprogrammed it. Is this correct?

It may not start at all. It definitely needs coding. If you don't then you could put the car in safe mode. Even on my old A6 C5 the ECU, injection pump and instrument cluster are all linked to the immobiliser. 

Yes, that was what he was referring to by his statement of it starting for a few seconds and dying, that is classic immobilizer behavior and exactly what I experienced when I replaced the ECU on a 2010 S4. His point was that since it wont start and die, its not the ECU, but Im not sure I buy that. Anyway, im going down there this morning to run some tests and see where im at. Ill post my results. .

If it's used it'll definitely need reprogramming, if it can be done. I've seen on eBay people selling ECUs with imobiliser deactivated. However I don't know if I'd trust this but just letting you know what's on eBay 

1 hour ago, Steve Q said:

If it's used it'll definitely need reprogramming, if it can be done. I've seen on eBay people selling ECUs with imobiliser deactivated. However I don't know if I'd trust this but just letting you know what's on eBay 

That's where I got th this replacement ECU. I bought them for Mercedes and they were about 120 bucks I bought bought them for an S4 which was only 100 bucks but the Q7 is few and far between especially TDI ones, I paid $500usd for it. I'm not sure if they're just that hard to find or if that many of them get water intrusion or go bad

39 minutes ago, what2be said:

That's where I got th this replacement ECU. I bought them for Mercedes and they were about 120 bucks I bought bought them for an S4 which was only 100 bucks but the Q7 is few and far between especially TDI ones, I paid $500usd for it. I'm not sure if they're just that hard to find or if that many of them get water intrusion or go bad

Hi I think the point that Steve was trying to get across was if you buy a refurbed unit with the immobiliser shunt built in its plug in and play, if its a used O.E. unit it will need programming, aside from that if its a refurbed unit and it still wont work there is definitely another problem further down the line, I.E. crank position sensor or cam position sensor.

Steve.

13 minutes ago, Stevey Y said:

Hi I think the point that Steve was trying to get across was if you buy a refurbed unit with the immobiliser shunt built in its plug in and play, if its a used O.E. unit it will need programming, aside from that if its a refurbed unit and it still wont work there is definitely another problem further down the line, I.E. crank position sensor or cam position sensor.

Steve.

Thanks Steve, spot on with what I was trying to say.