[Mark 1]

Hi guys. I'm new to this site so I hope I don't break any forum 'taboos' !
I have a problem with my 2004 Focus 1.8 petrol.
My car suddenly started to lose power and when I stopped , it ran really 'lumpy'
The breakdown guy towed me home and then performed a diagnostic test which gave the error code P0351 and said it indicated a faulty coil.
It turns out that I have no spark on cylinder number one so I checked the resistances in the coil and the HT leads which all seemed to be satisfactory (according to my Haynes manual) leading me to suspect a faulty plug.
When I removed the plugs , number one had an excessively large gap so I thought I'd found the problem but the problem was still there !
Still being reluctant to blame the coil , I did some online research and discovered that the coil in my wife's 2004 Fiesta shared the same specifications as the coil in my Focus , so I temporarily swapped them over together with the HT leads.
This made no difference at all ! My wife's Fiesta ran perfectly well with my coil and HT leads and my car still failed to run properly using her Fiesta's components so I'm now assuming that if my coil and HT leads work perfectly well in my wife's car , they must be perfectly good !
I understand that these coils work on the 'wasted spark' principle and that one coil operates plugs 1 and 4 and the other coil operates plugs 2 and 4. Surely this means that if plug 4 is firing , then the fault can't possibly be the coil , the wiring between the coil and ECU , or the coil 'switching' mechanism in the ECU itself (coil drivers) , otherwise NO plugs would fire OR only 2 and 3 would fire !
I'm totally baffled by this !
Any advice would be so very gratefully appreciated !
Many thanks ! 🙂

[Hometune]

Have you carried out a compression test? A bad intake or exhaust valve causes misfiring. I would strongly advise doing this before going any further.
Your coil and leads appear to be good as they work okay on the Fiesta.
Now for your ignition lesson! Yes, it is a wasted spark principle but, when number 1 is due to fire the main coil spark is triggered by the ECU. At the same time the wasted spark is triggered for number 4. This is not the same as the polarity reverses. So, yes if number 4 fires okay it does NOT mean that number one is okay due to internal failure of the coil.
Assuming the compression check is good - do all 4 and compare - anything less than 140 psi would point to wear and a reading of less than 100 means mechanical failure - then the fault is either:
a) the coil despite your testing
b) the plug to the coil which is a common fault and you can buy repair parts to eliminate this: https://www.ebay.co.uk/p/Genuine-For...ost/1125384593
c) the wiring between the ECU and the coil
c) the ECU itself - coil drivers. If suspected send off for repair to BBA Reman or ECU Testing (Google search to find them)

Bear in mind that swapping parts does not always definitely determine where the fault is. And Ford coils are notoriously poor but very cheap to replace.

[Mark 1]

Thank you so much for your reply ! I will certainly take your advice and carry out your instructions !
I will let you know what I find when I have completed your instructions.
Once again , thank you for your time. 🙂