I need some help, I have a 2003 8.1 Mercruiser Horizon engine that has no fire to the plugs on the even bank 2,4,6,8 there is fire on the odd bank 1,3,5,7 Not sure what to check on this matter.

Sounds like your 496 Horizon is either in the Guardian "mode" with a trouble code or you could have a problem with the harness or connections on that side of the engine. You should be connecting a scan tool to the engine to isolate your problem here, anything else is an expensive and time consuming guessing exercise.

Best Regards,
Ray @ Raylar

I figured it could be a coil or the ECM. The codes just show gardian mode and knock sensor

I doubt that 4 coil packs went out at once, It is in a security mode, more then likely

John jr

Check the plug that connects the 4 coils on that side of the engine. I have seen them get disconnected.

I 2nd that, check the harness, also make sure the wires haven't pushed out the back of the plugs, it will be connected but the wires can be pushed out enough not to make a connection.
Then check all of the grounds, make sure they are not loose or broken (even the small wires).
That is where I would start.