I have 2011 C300 Sport. The check engine light recently came on and OBD2 code was the notorious P0410 - Secondary Air Injection System Failure. I checked out 40 amp fuse and it was blown. I tested the M33 AIR pump by applying 12 volts directly to it and it was STUCK. I replaced the Air pump, 40 amp fuse (#32 in engine compartment) and Secondary air injection Relay (#P in engine compartment). Restarted car but air pump did not come on, fuse did not blow and check engine light remained on. I removed the relay and put wire across the relay switched terminals. Restarted car, air pump powered up and fuse did not blow, yet check engine light remained on. Problem isolated to air injection relay #P. It is not being triggered by the engine control unit.

Any suggestions as to what to check? I suspect one of the sensors going to the Engine control unit is not working.

Clear the code with a suitable code reader.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10