Oxygen sensor issue
 

So I keep on getting a lean fuel error code on the Justa for almost 1 month now and yesterday 1 got misfires on cylinder 1 and 2. The car will get the O2 sensor replaced today but I want to hold off with the coil replacement until the O2 sensor is replaced and see if it will go back to normal. I am assuming defective O2 sensors can cause cylinder misfires correct?

O2 sensor failure can cause all sorts of issues, misfire definitely. Hopefully you use OEM or similar high quality so you don't have to do it again soon. How many miles on this one?

138k kilometers. Limping and shaking now like the engine will fall-off.

Generally if an O2 sensor is bad enough to trigger misfires there will be a CEL accompanied by one or more (almost certainly more) error codes, consisting of one or more misfire codes, and a code that which points to a sensor. Now the bad sensor code may not be an active code. The bad sensor code might be a pending code.

You can replace the O2 sensors if you want. 138km (over 80K miles) can be enough miles the sensors are in need of replacement. (I had O2 senors go bad in my Boxster at around 80K miles.)

But the misfires are probably related to an air leak or possible insufficient fuel supply or pressure.

found the culprit as the coil(s). Now the question is do I replace just one bad coil or do I replace all of it.

A bad coil? Good catch.

A bad coil at 80K miles I would replace them all. Otherwise you replace the one being flagged now. And shortly thereafter the next worst coil, now the worst one, causes a misfire and you have to repeat the repair again. Then for the next cylinder and then for the 4th time.

But is it bad to replace just one at a time? I bought 4 new coils but I want to squeeze the life of the other coils unless it would throw a code or imbalance in the engine or wore out the new coil prematurely. It only takes 5 minutes to swap a coil anyway so it is not a concern for me to do it again besides I like getting my hands dirty once in a while.

Well, it is your car and you can do as you wish. My limited experience with one of my Porsches is new coils -- with no CEL or anything really pointing at a coil problem -- had the engine running better afterwards.

Squeezing a few more miles out of sub-par performing coils can result in less than optimum combustion which can increase engine deposits, wastes gas. While I like to get my money's worth out of consumables like plugs, coils and what have you as much as the next guy there are other things to consider. I do not regret replacing all 6 coils in my Porsche. Nor prior to that when an O2 sensor started triggering a CEL -- but the engine was not manifesting any signs of any problems -- and I had all 4 sensors replaced (at 132K miles) and the engine ran better afterwards I had no regrets that there might have been, probably was, some more service life left in the other 3 sensors. But I don't work on my cars and to take the car in to have one sensor replaced at a time... No thanks.

Fair enough will just swap them all out and keep the old working one as back-ups just in case something happens and I can momentary swap one or two out while waiting for new sets to come in in the future.