Cause
Failed air assist control valve
The valve controlling air used to assist fuel atomisation at the injector, failed so the assisting air is no longer metered. Air-assisted injection exists to improve atomisation when the engine is cold, when fuel does not vaporise readily and a coarse spray condenses on the port walls instead of burning.
So its failure is a cold-running fault that disappears once warm. Poor starting, a rough and smoky first minute, and stalling on a cold morning — with an engine that behaves perfectly by the time it reaches a workshop. That disappearing act is the diagnostic signature and the reason the complaint is so often dismissed.
Check the air passage as well as the valve. These systems draw air from upstream of the throttle through small passages that carbon up, so a healthy valve can be feeding a blocked path. Command the valve and listen or feel for operation, then confirm air actually reaches the injector — a valve that clicks correctly while nothing flows is the commoner finding.
Typically a moderate job.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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