مشعب العادم متشقق

The exhaust manifold cracked, admitting outside air into the exhaust and letting gas escape. Manifolds crack from thermal cycling — heated and cooled through a wide range every journey — and casting stress at the flanges, which makes it a wear-out failure on an older engine.

The air it admits is what reaches the diagnostics. An upstream oxygen sensor sees more oxygen than the engine produced and reports lean, so the module adds fuel and the engine genuinely runs rich. A rich-running engine with a lean-reporting sensor is the signature. Listen cold: a crack is loudest before the metal expands and closes it, which is why a tick that fades as the engine warms is the classic sign and disappears by the time the vehicle is on a ramp.