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Peter Zavarelli
Peter Zavarelli, Auto Mechanic
Category: Car
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Experience:  Technical Consultant at Just Answer
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Mondeo Mk4 petrol. Intermittent failure to start. Turn key,

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Mondeo Mk4 petrol. Intermittent failure to start. Turn key, instruments etc. light up, turn to start, " engine immobiliser activated" flashes up briefly, but nothing happens. Fiddle about a bit, locking, unlocking, slamming doors, after 5-15 mins. it starts normally. Seems to only happen after running long enough to warm up. but not every time.
Main agent hasn't been able to diagnose fault, but suggests new PCM at £1200 + fitting.
I suggest this is just a guess.
Any ideas?

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.My info is somewhat limited on your model here but I had seen no one else has responded and would like to see if I can help here.If at any time you feel that I cannot help with this please let me know.

From what you have described here this sounds like your vehicles theft system is engaging keeping the engine from starting intermittently.It can very well be the PCM intermittently at fault causing this issue as the electronics inside of it can act up causing issues like this here.I have also seen issues with the immobilizer sensor that reads the key cause this fault intermittently with it needing replaced.This is probably most common.

If the shop/agent has not checked for any codes in all computers in your vehicle this needs done first.There should be at least one code set that will help to diagnose and pinpoint this fault here.The code can lead to the component at fault needing replaced.

But the sensor that reads the key at the ignition is usually the most common cause although I wouldnt rule out the PCM intermittently acting up as this is possible as well here.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated ( should be a link to click on here or above).You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 1 year ago.
Hi Pete. Thanks for that.
The main Ford Agent in Inverness had the car for a week, said:" no codes on system currently". Doesn't make sense to me. PCM still sounds like a guess.
Any idea of the cost of an immobiliser sensor? How complicated to fit?
Cheers Peter.

Your welcome

If it is the PCM or sensor at fault once replaced they usually have to be programmed by a shop or dealer.The cost can vary running anywhere from around $150 to over $500 all depending on the facilities prices.

Let me know if you need anything else and feel free to rate my help here or above ( should be a link to click on ) as this is how I receive credit for my time,this can be done at any time and you can still come back here and reply if needed.Your 5 star rating is appreciated

Thanks Pete

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