P0301 instead of P0300: the cylinder number is a head start

A cylinder-specific misfire code has already done half the diagnosis for you: the problem lives in, or is delivered to, cylinder one. That collapses the suspect list to that cylinder's ignition, its injector, and the cylinder itself — in that order of likelihood and cost.

Make the parts prove themselves before buying anything. Swap the coil with a neighbouring cylinder, clear the code, drive. If P0302 appears, the coil carried its fault along and you have your answer for the price of ten minutes. The same swap logic works for the plug and, on many engines, the injector.

While the plug is out, read it — it is the cheapest borescope you will ever use. Oil on the threads says valve-stem seals or rings; a fouled black tip says it has been misfiring rich for a while; a chalk-white tip says lean, and the misfire may be a symptom of an air leak rather than an ignition fault.

If the code stays on cylinder one after ignition and injection have been swap-tested, it is time for a compression or leak-down test — the mechanical causes (valve, rings, head gasket) are the expensive third of the list, which is exactly why the swaps come first.