Toyota Repair: 2001 toyota corolla idle problems: follow up

Question
Yes the check engine light is on. I found a neighbor that had a diagnostic tool so I checked it and I got the codes P0300 P0303 and P0304 what should I start to replace?

Answer
Hello, there are two fairly common problems, spark plugs and ignition coil packs, remove the plastic cover from the engine and inspect the engine harness wires carefully including the fuel injector connectors and wring for any signs of damage from rodents. Remove the coils and inspect the insulators for burning or arcing, remove the spark plugs and replace them if in doubt. Clear the codes by disconnecting the battery for about a minute, drive it and if the same codes come back remove the #3 coil and switch it with #1, clear codes and drive it again, the check engine light will come back and if the codes now show P0301 and 304 replace both #1 and #4 coil packs.