Fixing a Ford Focus/CMax's auto-start/stop system - it's all in the battery!

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This foxed me for ages. And I'm hoping that relating the tale will help someone else. We have a Ford CMax (2015), but the same systems apply for the Ford Focus, on which the CMax is based. We bought it in 2018 and the auto-stop-start system worked fine for a couple of years. As a reminder, this auto-stops the engine when stationary in traffic and when the handbrake is on (and gear in neutral, obviously). Then, when you engage a gear and release the handbrake, the engine is auto-started and you're off. A nice fuel-saving measure, in theory.

Alas, around 2020, the auto-stop-start system ceased to work. The green 'A' above was greyed out and with no explanation. Research online pointed to this system not working in very cold or hot weather, or when the engine's not warmed up enough, or if you have all the lights and wipers on. Sensible reasons. But not applicable in our case - it remained a mystery.

There was a clue in that 'low battery state' was also listed, but I gave that no thought, since the car was working just fine.

Then, a couple of months ago, I had the car not start first time, just a gentle chug rather than firing up properly. It was only one time in 20, and I went chasing obscure reasons like EMF interference with the immobiliser when trying to start in an industrial or retail area, with lots of big machinery. Heck, I might as well have investigated nearby UFO sightings.

Then it occurred to me to put the two car symptoms together. What if the battery was indeed suspect? Looking back through my service history, there was no evidence that the battery had ever been changed! In our case that means the battery was between 8 and 9 years old. Oops. Even though most things still worked just fine, clearly the battery was long overdue replacement (manufacturers quote 5 years as an upper limit!)

So... I bit the bullet and had the battery replaced by a professional - it's not a DIY job on modern engines, as there are ducts and pipes and so forth to remove in order to even get to the battery tray.

The car started fine with the new battery, but the auto-stop-start was still disabled. Again, looking online, it seems that the usual arcane reset procedure is five presses on the rear fog-lamp button followed by three on the hazard button. Nothing flashed on my dash to indicate that this had been accepted and thought "Oh well".

But then, down the road, I stopped at lights. And lo and behold, the engine was auto-cut. And restarted, and all has been well since!

PS. I did try that button procedure a few years ago, i.e. with the old battery, and it did nothing. Because the latter just wasn't up to it, I think.

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