On my GF 2002 Protege LX 2.0L.
I had the P0300 & P0421 codes. I changed the plugs, the coilpack and it fixed the P0300. Then I changed the Cat (It was all in pieces inside), and it cleared the P0421.
Then, after 2-3 weeks, I got this P0401 code (EGR), so I erased it to see if it would come up...