ok bought a MC 2004 ran for for prev owner for a bit ... changed lock cylinder and sensor "used " after correct program sequence ram great!! on off on off about 15-20 times .. anyway let sit for 40-an he would not go.. Idea's anyone?? need help I don't want to admit to my wife I got a lemon ... pls help

 

So the original problem was ...no start with a security message in the dash? Probably a bad ignition lock cylinder HOUSING. A common problem lately as our cars age.

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I had security problems too, I changed both the lock and housing to be sure I got it right, no down time wanted. I'd invest the extra money and do the housing too, as that's much easier than the lock.

 

Hey x, can you please answer my pm?

 

Not trying to make you sound like an idiot, but have you checked for other problems? Too often we center out efforts on the last part we fixed that we knew was bad.
You said that you replaced the lock cylinder with a used one and had it programmed, and it worked fine. So, either you just had bad luck and the used one went bad, lost the programming, or you have another problem. Make sure you have battery voltage and good connections. Also, make sure your getting the power to the starter. Just a thought, where you don't know the vehicle, before spending a ton of money on another key cylinder double check for other issues.