P0420 means the engine computer has judged the catalytic converter on Bank 1 to be cleaning the exhaust below the required efficiency, based on the oxygen sensors before and after it. It is one of the most common check-engine codes on any gasoline vehicle. The definition is the same on every make — what differs is the likely cause and fix on your specific engine.
By far the most common cause. The converter's precious-metal coating loses efficiency with age (typically 80,000–150,000 mi), and is poisoned faster by oil or coolant burning, leaded fuel, or repeated short trips that never bring it to operating temperature.
The rear O₂ sensor is what the computer uses to grade the catalyst. A drifting or slow sensor makes a healthy catalyst read as inefficient — a frequent false trigger.
A cracked manifold, blown gasket or leaking sensor bung lets extra oxygen in, skewing the readings before the exhaust ever reaches the catalyst.
Unburned fuel from a misfire (see P0300) or a fuel-trim fault overheats and coats the catalyst, degrading it — fix the root fault first or the new catalyst fails too.
Occasionally a manufacturer re-flash tightens the catalyst-monitor threshold. Actual PCM hardware failure setting P0420 is uncommon — rule out everything above first.
Related codes: P0430 — same fault, Bank 2P0135 — upstream O₂ heater, B1S1P0141 — downstream O₂ heater, B1S2P0300 — misfire — can damage the catalyst
In this family: P0421 Warm-up catalyst efficiency, Bank 1 · P0430 Catalyst efficiency below threshold, Bank 2 · P0431 Warm-up catalyst efficiency, Bank 2
The definition is universal, but the diagnostics are engine-specific. Open your vehicle for the codes and fixes documented for it:
Short-term, usually yes — a mildly inefficient catalyst isn't a safety risk. But it will fail an emissions test, cost you fuel economy, and can worsen over time, so don't ignore it.
Only if the catalyst is actually the failing part. If a downstream O₂ sensor or an exhaust leak is feeding bad data, a new converter alone won't clear the code — confirm the cause first.
Clearing it only turns the light off temporarily. If the underlying cause remains, the code returns after a drive cycle or two.
Almost never. P0420 is a catalyst-efficiency code; the fix is nearly always the converter, an O₂ sensor, or an exhaust leak — not the computer. Be wary of anyone selling you a module for it.