What should I do if the EPC and check engine lights came on again?

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I have a 2015 Jetta 2.0 no turbo with just over 100,000km. The epc and check engine lights came on, with OBD showing error codes P0301 and P0304. I changed all 4 spark plugs, which removed both lights and resolved the error codes. After a few 100 km now, both lights came on again, with OBD showing P0304. The first time the lights went on while I was driving (at about 60km/h), the second time the the lights went on while idling (about 5mins after starting the engine). What could be the issue?