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Mk4 Mondeo TDCi ECU / immobiliser issues

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Hi All,

We have a Mk4 Mondeo 2.0 TDCi. Recently it was left untouched for just over a week. When I went to start it, it wouldn't - immobiliser active apparently. A bit of googling suggested a low battery *could* cause this - and we've known the car to have issues with the battery draining for no apparent reason. So I tried jump leads. No joy... So we got a mechanic in. 

Long story short, it seems I may have fried the ECU with my jump leading... However, the mechanic DID manage to start the car .... albeit for a minute or so before it cut out again. Initially he reckoned the ECU was dead - but now seems to think he can fix it (the ECU) ... and we could then avoid the expense of replacing the ECU and trailering the car to Ford for reprogramming, *if* we can find the spare key (which of course we can't!) - as he thinks the "main" key is dead (moreso than just a new battery). But since it did start for a minute or so, can the ECU be that dead?

I can understand how the jump leads may have frazzled the ECU - but I don't get why the "main" key is now useless? How could the jump leads have killed a remote keyfob??!!

Bracing myself for the situation that the ECU cannot be fixed and needs replacing - I've spoken to a local Ford dealer, who admitted he wasn't sure, but seemed to think we'd need both keys if we did end up needing to recode it all - surely that can't be right - we can't be the first people to be in this situation?

Any idiot proof advice welcome!

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  • 1 month later...

all a load of rot!


Basically, if the battery is low, then the voltage destabilization can cause fluctuations in the instrument panel and the key code. Worst case scenario is that the key has been lost from the memory of the instrument panel, which means the car no longer understands the key you have.


Have you replaced the battery

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