P0420: sprawność katalizatora — od czego zacząć

P0420 (and the mirror P0430) usually indicates catalyst system efficiency below threshold for bank 1 (or bank 2). It does not automatically mean the catalytic converter is dead—false positives happen.

Common checks before replacing the cat: exhaust leaks upstream, degraded oxygen/air-fuel sensors, misfires or rich mixtures that poison the catalyst, and confirming the code returns after a proper drive cycle.

Live data helps: compare O2 sensor switching pre- and post-catalyst, watch short- and long-term fuel trim, and look for related codes (e.g. misfire or fuel system lean/rich).

If you only commute short trips, incomplete readiness can mask intermittent issues; a highway drive after repairs may be needed to validate the fix.