Hi, all.

Just posting to get an alternative viewpoint on this.

I’ve got the error codes:
P0420 - Efficiency Below Threshold
P0301-8 - Cylinder Misfire.

My plan is to replace to o2 sensor (if I can, am yet to research) and if the issue is still there then I’ll replace the ignition coils.

 

I think you have it backwards. Replace the coil, do a hard reset and both codes should go away. But do some research on coils, there are a lot of poor quality ones out there.

 

The O2 sensor message only comes up when driving on the motorway. The first time, it was the only code.
Only recently both codes came up.

My thinking is to replace the sensor as it's the cheaper fix, and if that does not fix the problem then change the coils?

 

So: misfire codes for all 8 cylinders, ne? have the secondary tensioners been replaced? Pull a cam cover and check for oil in the spark plug hole . . .