A plugged Secondary Air Injection Port on a Mercedes Benz M112 V-6

So the air pump is not accomplishing it’s task of heating up the three way catalyst. How does DME (Digital Motor Electronics) know any way?

Programmed into the computer is a self test of the air pump. Each time the car is started DME at some point in the drive cycle will turn the air pump on to see if it’s working.  How it sees is interesting.

Part of the fuel management system are oxygen sensors or on newer cars AF (air/fuel) sensors or broad band oxygen sensors report back to DME.  By sensing the amount of oxygen in the exhaust gases these sensors allow DME to make adjustments to the air fuel mixture going into the engine for the most efficient performance under all conditions.

The O2 sensors also tell DME if the air from the air pump is reaching the catalyst. If the secondary air injection system is working properly DME should see a drop in voltage in a conventional O2 sensor.  On the newer A/F sensors the technician will need to access fuel trim data to see if the fuel system is trying to adjust when the air pump is artificially triggered.

So the air pump pumps air but there are some mechanical controls in the way of the air actually getting to the exhaust side of the equation. There is a vacuum switchover valve (VSV) and a vacuum operated “Dump Valve” that when vacuum is applied by  way of the VSV allows air to enter the exhaust through a check valve which keeps exhaust gasses from returning through the “Dump Valve” damaging it and the pump.

So the long and short of it is, the DME turns on the Air pump, the sensors sense no change indicating A Secondary Air Injection malfunction or low flow and DME turns on the Check Engine Light. Now what?

Dig in.