
P0420 / P0430
Exhaust · Catalyst
The Rattle That Became a Roar
Catalytic Converter Failure
Marcus called from the breakdown lane south of Exit 47. He described a metallic rattling that had started three days earlier — intermittent, louder on acceleration. By 10 PM it had become a sustained roar. The check engine light had been on for two weeks.
Tow arrived at 11:34 PM. By midnight the car was on our lift. The OBD-II scanner pulled a P0420 — catalyst system efficiency below threshold — alongside a P0430 on bank two. The heat shield had separated and the substrate inside the converter was fragmenting.
We quoted before we touched anything further: one aftermarket direct-fit catalytic converter for his 2017 Accord, two hours labor. Total: $680. He approved by text at 12:18 AM. Car was ready at 7:15 the next morning.
What P0420 actually means
Your catalytic converter oxidizes unburned hydrocarbons. When efficiency drops below 95%, the downstream O2 sensor sees the same waveform as the upstream one — that mismatch triggers P0420. Driving on it won't strand you immediately, but the fragmenting substrate can eventually clog exhaust flow and kill the engine.


