P420 After New catalytic changed on 1998 Mazda 626

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I have p400 code for about 2 years. This Feburary i changed catalytic because it got rust damaged. After that i got 3 times P303 which all happend on i starting drive on raining day. Recently i found there comes a new code p420. What are the possible reason can make these problems? I am going to change ignition coil. Is that gonna help?
(1) Answer
P0400 is an EGR failure code. There's something wrong with your EGR system. P0303 means that you have a misfire on cylinder #3. That it happens in the rain suggests that the sparkplug wire or insulator boots are leaky and shorting to the engine in the wet. Check for arcing sparkplug wires. P0420 indicates that the catalytic converter efficiency is below specification. Sounds like 3 separate problems to me. Address them one by one. Probably it's not an ign coil...