BMW 279C — Secondary-Air System 2

No driveability symptoms. The driver does not notice any performance difference. The US-market emissions warning lamp (MIL) illuminates; ECE-market vehicles may not illuminate the MIL for this fault. A CC message is displayed. The vehicle will fail an emissions inspection with this code active. Cold start exhaust emissions are elevated.

1. Leaking secondary air injection hoses. The rubber hoses connecting the air pump to the check valve and exhaust manifold port become brittle with age and heat exposure, developing cracks that leak air before it reaches the exhaust. This causes low differential pressure readings.
2. Failed secondary air check valve (combination valve). The one-way valve that prevents exhaust gases from flowing back into the air pump can stick open (causing exhaust gas leaks and pump damage) or stick closed (blocking airflow). Carbon buildup from exhaust gases is the usual cause.
3. Blocked secondary air injection port in the exhaust manifold. Carbon deposits can clog the port where air enters the exhaust, especially on engines with higher oil consumption. This causes high differential pressure readings.
4. Failed secondary air pump. The pump motor may be weak, seized, or have melted internal components from exhaust gas backflow through a failed check valve.
5. Leaking exhaust manifold gasket near the secondary air injection port. Exhaust leaks in this area affect the pressure differential.

1. Inspect the secondary air injection hoses on bank 2 for cracks, splits, or disconnected connections. The hoses run from the air pump output to the check valve and from the check valve to the exhaust manifold. Replace any degraded hoses.
2. Check the secondary air check valve operation. Disconnect the hose from the exhaust side and attempt to blow through the valve in both directions — air should flow freely toward the exhaust but block in the reverse direction. If it flows both ways, the valve is stuck open. If it blocks both ways, the valve is stuck closed.
3. Activate the secondary air pump using ISTA or INPA. Listen for pump operation and feel for airflow at the check valve inlet. Weak or no airflow indicates a failing pump motor.
4. If the pump runs and the check valve works, inspect the secondary air injection port in the exhaust manifold for carbon buildup. This may require removing the check valve or injection pipe to visually inspect or probe the port.
5. Check for correlating fault codes — P0491 (bank 1 insufficient flow) appearing simultaneously suggests a pump or shared hose issue rather than a bank-specific problem. Relay control faults (2793/P1414, 2794/P1413, 2796/P1411) suggest the pump is not being activated at all.

Replace cracked or degraded secondary air hoses. Replace the check valve if it is stuck or leaking. Clear the carbon-blocked injection port if accessible. Replace the secondary air pump if output is weak or absent. Clear the fault code and verify the repair by performing a cold start (vehicle must be fully cooled, engine temperature below 38°C) — the MIL should not return after two consecutive qualifying cold start events. This is generally a DIY-accessible repair, though accessing the exhaust manifold injection port may require removal of heat shields and nearby components.