When low-efficiency catalyst codes PO420 and PO430 appear, the last place that most technicians would consider would be a defective air filter seal or damaged/distorted airbox, allowing air filter by-pass. Often, the mentioned codes result in oxygen sensors and the catalytic converter getting replaced, which may be a short term expensive fix, especially when the condition recurs. Considerations for the stored codes normally include engine misfires, excessive oil consumption, lean fuel mixture, rich fuel mixture, weak ignition, damaged or defective oxygen sensors, any of which could result in the aforementioned codes. Contaminated air is seldom considered.
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