2008 GMC Sierra with p0121 & P0304

[quote=”mbahzad” post=141718]First of all, whats the difference between the code with and without “p” in front of it?[/quote]
None. Perhaps a peculiarity of your scanner’s software.

secondly, can one code cause the other?

Nothing is impossible, but not very likely, since the misfire code is cylinder specific.

which code should I work on first?

The throttle position sensor (TPS) code is more severe because that affects the engine management (timing and fuel mixture) system. I would go after that one first. You need to find out first if the TPS is adjustable. If so, the adjustment procedure should be in your service manual. If it is not, then there should be some resistance or voltage tests that can be done (also in service manual) to prove whether it needs replacement or the code is caused by something else. Sometimes a ridge of deposits will build up around the inside of the throttle body and prevent the throttle plate from closing all the way. This condition can set a TPS code when there is nothing wrong with the TPS. Before doing anything with the TPS, you may want to inspect and clean the throttle body. This does not require removal of the throttle body. This video may help:

When it comes to misfires, can a simple spark plug change fix the problem?

I have seen many misfires cured by changing spark plugs (and wires, and distributor caps/rotors). Whether it will cure yours, I cannot honestly tell you. It wouldn’t hurt to pull #4 spark plug and check it out, though.

The big thing here is to not fall down the rabbit hole of indiscriminately swapping out parts without confirming a diagnosis. If tune-up items are old or the engine is overdue for a tune-up, then ok go ahead and change those, but don’t just throw parts at a problem hoping for the best. A diagnosis will be especially needed for the misfire code, because misfires can be caused by so many things.