anyone done this lately?
and is there anything else I should replace while I'm there?

Thanks

since it went out I also got a 303, oxygen sensor, would this be causeed from the gasket being out??

 

On underneath of valve cover there are a seal for each spark plug. I would definitely replace them when the valve cover is off. I would replace valve cover gasket and see if the problem goes away.

 

Definitely replace the spark plug cylinder seals.

 

The gasket has nothing to do with your O2.

Reset the ECU, if it comes back with the same code, replace it.

 

Don't forget to clean the surface of the old silcon sealend, and replace it in the same places with new stuff.

By "went out", what happened? Just leaked?

 

My car feels like it is working too hard, and mileage started to go down drastically.
So, i checked my plugs and found some oil in the threads, and inner was burning dirty.
And I also got a front O2 sensor code.
Would these affect performance and mileage??

 

Significantly!