P0420: OBD-II code P0420 means the bank 1 catalytic converter isn't reducing emissions enough — the downstream O2 sensor reads nearly identically to the upstream one.

Symptoms

• The check engine light is on, but the engine typically runs and drives normally with no immediate performance issues.
• Fuel economy may drop slightly, and a sulphur or rotten-egg smell can appear from the exhaust on some vehicles.
• The vehicle will fail an OBD-II emissions test until the catalyst monitor completes and reports 'ready' with no pending code.

Common causes

• A worn or damaged catalytic converter that has lost its ability to store and release oxygen — the most common cause on high-mileage vehicles.
• A failing downstream O2 sensor reporting inaccurate switching behaviour, mimicking a bad catalyst on the scan tool.
• An underlying misfire, rich/lean fuel condition, or oil-burning issue that damaged the catalyst — always fix the root cause first.

What to do?

Read all stored codes and address any misfire, fuel-trim, or O2 sensor codes before condemning the catalyst. Read live upstream vs downstream O2 voltages: downstream should stay steady around 0.7 V and not switch with the upstream. Replace a bad O2 sensor first (much cheaper than a catalyst). If sensors are good, install an OE-spec catalyst to keep the code from returning.