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Battery blunder! Help would be appreciated

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Hi guys,

My car is a Ford Focus 1.6 petrol, 2007

I made a very stupid mistake yesterday. I fitted a new battery, and I connected the battery the wrong way around (+ to -, and - to +). There was a loud whirring noise and I quickly disconnected it and realised my stupidity.

I've now connected the battery the correct way around, and now the car won't start up. The electrics seem to be still working, the central, locking, dashboard, eletric windows are all fine. However, the car won't turn over and the dash is showing 'engine system fault'.

I did some reading online, and some people are saying 'it will have fried everything' and others say there are fuses to protect the components of the car including the ECU.

Has anybody got an ideas about what I can try?


Thanks very much in advance

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Fuses are only intended to protect the electrical system from overcurrent. Reversed polarity is not protected by the fuses in any way.

There is a huge chance that the reversed polarity damaged the PCM. 

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Not what I wanted to hear, but thanks for your advice! Sounds like my best option is to get it booked in to a garage for them to confirm this.. Am i right in thinking that if it is the PCM, then I'm looking at hundreds of ££s to sort it?

Thanks again mate

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32 minutes ago, JW1982 said:

There is a huge chance that the reversed polarity damaged the PCM. 

Well, protection against wrong polarity is cheap and simple and also kind of basic matter in electronic designing.
And as said, it's not implemented by fuses, but it's internal feature of module.

I doubt, that the manufacturer (Bosch, Magneti Marelli, Lucas, etc.) of car electronics would left wrong polarity or overvoltage protection away from their modules.🤔

There are so many possible faulty devices from broken wire, fuse, relay or sensor to electronic module(s) - or maybe the reverse polarity protection just operated as it's designed.

Also, error codes may help for troubleshooting. I prefer to book the garage.

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Have you checked the high ampere fuses in the engine fuse box? It's worth checking them and seeing if there is any 'basic' reasons for the problem. 

As @AR7530v6 says it's unlikely but not impossible that any part of the ECU is fried. 

Check all the fuses in the engine bay and then move on to the fuses in the cabin, although it's less likely one of those has blown. It's worth doing the simple stuff first.

Are you able to connect a fault code reader up?

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