انسداد ممر الهواء الثانوي بالكربون

The drilled passages carrying injected air into the exhaust ports, blocked with carbon so the air cannot reach the exhaust stream. The pump and its valve can be in perfect condition — the system fails at its outlet, which is the part nobody can see and which no electrical test reaches.

It is a designed-in vulnerability rather than neglect. The passages open directly into the exhaust port, so every combustion cycle deposits carbon at their mouths for the whole life of the engine while the system itself runs for barely a minute a day. Blockage is gradual and the flow codes appear long before anything else is wrong.

It is distinct from a seized check valve, which sits between the pump and the passage and fails as a mechanism rather than as a restriction — the two produce the same insufficient-flow code and want different work. Confirm which by measuring flow with the valve removed: air that passes the valve freely but still will not enter the exhaust locates the blockage in the head. Clearing the passages usually means removing the manifold, which is why this fault is often the point at which the system is written off.