[BrendanP]

My 2015 1.6 diesel has recently begun to not start first time. The engine isn't cranking but there is a whirring sound as though the starter motor is spinning. If I stop and try again it usually starts as normal. I know the solenoid should push the starter pinion along the shaft splines to engage with the teeth on the flywheel, and the contact in the solenoid should close to energise the motor. Does anyone have experience in overhauling the starter motor? My thoughts are that it could be the pinion is sticking on the motor shaft, or the motor brushes are worn down. Any ideas?

[The Gonz]

That little bit of boost to get the solenoid to engage the pinion could be missing from an otherwise healthy battery.
Have you checked your battery voltage at rest and under load?

[BrendanP]

The battery is a Yuasa YBX9096 AGM, and it's 3 years old. The engine cranks over fast enough so I don't think the battery is the problem. I can measure the volts when I have a helper.

[The Gonz]

I'd still be betting on electromechanical. If the voltage is good, is it actually getting to the solenoid at full potential or is there a voltage drop along the way?
Weak termination? Compromised cable? Can the solenoid be dismantled and cleaned?

[BrendanP]

It did it a couple of times this morning during auto-start. That's a real pain when moving off from traffic lights with cars behind you.

[The Gonz]

Ah, new information, you have autostart! That complicates it somewhat but also clarifies it. The autostart logic is also a player now.

[Greyhound]

... I can measure the volts when I have a helper.

Did you get to do a battery voltage check?

I assume you can switch off auto-start for journeys for the time being to avoid slow getaways.