Cause
Split or disconnected air hose
A hose in the secondary air injection system split or blown off, so pumped air escapes before reaching the exhaust. The system runs for barely a minute after a cold start, which is what makes this so hard to catch: by the time anything is connected, the pump has stopped and the hose is not under pressure.
The module detects it as insufficient flow, because the oxygen sensors do not see the expected lean shift when the pump runs. So the code names flow while the pump, the valve and the passages are all fine and the air is simply going somewhere else.
These hoses run hot and are frequently a moulded rubber or plastic pipe that hardens. They split on the underside of a bend or release from a stub whose clamp has relaxed. Watch and listen during the first minute of a genuine cold start — that is the only window in which the fault is live, and a hose blowing off is usually audible when the pump is running.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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