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Service Engine light, Diesel, after high rpms on highway

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Hello folks,

I have a 2017 Focus Estate 1.6 TDCi that will squawk "service Engine Now" and go into limp mode, sometimes,

- after I've been running at highway speeds for a while

- mostly after a downshift that takes the tach over 4000.  I think once during engine braking in 3rd after I'd been over 4000.

Always on the next startup the warning will disappear.  Reputable local Ford specialist pulled a bunch of random faults mostly related to "loss of electrical connection".  Sent me on my way.  Here's what I've heard:

- someone told me when it first happened that the blue wiring harness connector under the plastic cover near the battery can come loose, causing brief loss of electrical signal

- local shop's guess was that it was under-batteried, and the original battery should be replaced to prevent momentary low voltage events.  But it passed their battery test.

Neither explanation makes sense to me, unless the combination of engine compartment heat and some high-rpm resonance shakes the connector loose.  But it hasn't happened during high-rpm operation at lower speeds in town, no matter how long the car has been running.    

I can putter around for months without it happening, but I want to drive it vigourously every now and then without the hassle.  And of course it will happen when I go to sell it, which I'd like to do with a clear conscience. 

Anyone else experienced this?



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Check the BCM (combined computer module and fusebox) located around the passenger footwell / glovebox area for any corrosion on the wiring connector pins.

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  • 4 weeks later...

BCM connections clean...  

After doing some driving experiments, it seems the service Engine Now light will come on after any high-rpm engine braking.  Will happen in a lower gear going down steep hill at low speeds, no throttle, just engine braking upward of 4000 rpm, even if there has not been an extended highway run beforehand.  

In that driving scenario, there is no fuel being delivered, and absolutely nothing should be happening except mechanical bits moving.... I wish I understood modern Diesel control systems to even guess at what could throw a code.  No fuel = no emissions = no exhaust gas recirculation? 

High rpms could mean alternator over-voltage I suppose, which might confuse the electrics.  Hard to believe there's a strange resonance that would interrupt electrics at 4000 rpm.  It doesn't happen during high-rpm upshifting.

Will update if I ever find anything!


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You need some Ford specific diags on this.  No point 'guessing' what codes are being thrown.

Do you still have the codes Ford specialist provided?  If not, it would be well worth getting Forscan to read the Ford specific codes yourself.

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