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The starter worn — brushes, bushes or solenoid contacts — so it cranks slowly or intermittently. On a stop-start vehicle it works far harder than on a conventional one, since it is used at every junction rather than once a journey, and it is built accordingly and still wears.

Slow cranking is the fault rather than no cranking, and it matters diagnostically: an engine turning below its minimum cranking speed may not start even with perfect fuel and spark, and the module may not see a valid crankshaft signal. A no-start with a slow crank is a starting-system problem, not a fuelling one.

Distinguish it from a supply problem first, because they present identically. Measure voltage at the starter while cranking — a healthy supply with slow cranking is the motor, and a supply that collapses is the battery or its cables. Burnt solenoid contacts give the classic single loud click with no rotation, and on many starters they are a serviceable part rather than a reason to replace the unit.