Disco has been running like a well tuned beast the last year or so, but very recently I started noticing a lot of soot accruing around the tailpipe tip. Then I got a P0300 and P0301 as pending codes today. The codes seem vague, but perhaps with the sooty pipe that might point to something more certain? MAF and O2 sensors were replaced in 2014, and 75K miles ago. Wonder if those might be where I start looking?

 

How do the spark plugs and wires look?

My D2 was recently kicking up intermittent cylinder misfires and O2 sensor codes. The coils, plugs, wires, and O2 sensors had been running 75K+ miles.
Replaced the O2 sensors because easy enough. No change to codes.
Then a coil was starting to go bad. While replacing those along with plugs and wires, I found one of the spark plugs was realy worn, and others showed wear. Since I've replaced coils, wires, and plugs no more misfire or O2 codes.

 

Maybe focus on cyl #1 (P0301). The random code (P0300) is often generated when any single cylinder is misfiring. Check items mentioned above, but always a good idea to monitor O2 trends.

 

The wires are relatively new, but that's a good idea, I'll check that out. Should be easy now that the coil packs are relocated! haha.

 

An 02 sensor want cause a miss-fire. Check the plug wires and make sure none are run too close to the secondary air tubes if equipped. I had one get burnt on a SA tube and was arching causing the miss-fire. The easiest thing to do is swap the plug and wire from Cylinder 1 to 3 and see if the miss-fire follows the plug and wire.

 

Usually easier to swap 1 and 2 as they are closer to the same length.

 

Plugs, coils, O2 sensors, and MAF are all due for a change with that many miles (or at least a cleaning for the MAF).