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My 04 GXP with 62K miles. I filled it up and it was running fine. Then then next day I took it to the local Chevy dealership to check why the oil pressure was zero (they said the sending unit was bad and wanted over $300 to repair so I told them I would do it myself). It was idling and running fine when I dropped it off. But they had to replace the low pressure AC line due to a leak and replaced both oil cooler lines as they were leaking, and both front hubs. I have an extended warranty so that covered a lot of the cost. I picked up the car yesterday and within 2 miles the SES light started flashing and it was running rough. I scanned it at home (3 miles from the dealer) and the only codes that came up were P0300 and P0520, the second has to do with the oil pressure switch. I started the car up again today and it is still running rough with the SES light flashing, so it is doing this with the engine cold or warm.

There is no associated single cylinder misfire code.

Is there anything they could have done to the wiring or something else that can cause this because it never happened before I left the car there? Or is it an entirely new problem?

Take it back.

There is some good news I can think of. That is, you're probably getting sufficient oil pressure. Why? P0520 is triggered when oil pressure is greater than ~5psi (if I recall), and is a sensor signal circuit fault. My guess, When disconnecting the plug, they were too forceful and messed up some terminals or damaged the plug.

How would it correlate to a P0300? Not a clue. The only plase these two would cross would be in the PCM. That's the bad news if that is the issue.

The delaership is telling me they ran some tests and the misfires are coming from cylinders 5 and 6. Since I never had a misfire before it left the dealer seems unusual that 2 coils or plugs would go bad at the exact same time, both on different banks. Now they want to swap out the coils with other cylinders to see if they are bad. They said the mechanic checked all the vacuum lines and wiring in the areas that they worked and did not see any damage.

S.O.P.