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I've had the check engine light come on twice recently with secondary air injection as the fault. It is a 96 with 65k miles. Any info on what is the cause is greatly appreciated.
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2003 SL500, 2003 SL55, 2005 S55, (2) 1991 300SLs, 1994 E320 cabriolet
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Generally every trip the engine management controller performs a brief test of the air injection system. The exhaust mixture is checked, injection is started, and the mixture is checked again. To pass the test, the 2nd reading must be at least 25% leaner than the 1st. If the test fails it is remembered, and if the next test on a subsequent trip also fails, then the check engine light (CEL) is set.

Often a CEL for air injection occurs even when the system is functioning perfectly. What happens is the oxygen sensor has aged and is unable to register the 25% lean condition necessary for passing the test. In this case the only solution is to replace the sensor.

You can probably tell if the air injection is working. The pump should be running just after a cold start. Also, you will notice a vacuum line running to a control valve close to the pump. There should be vacuum on this line while the pump is running and no leaks in the control valve.
 
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