This code comes on when your vehicle's computer detects that the catalytic converter is operating below its efficiency threshold. In plain English — your vehicle has a special filter built into the exhaust system that cleans dirty air before it exits the tailpipes. P0420 means that filter is either too dirty to operate efficiently or is failing mechanically.

What Causes P0420?

Can I Still Drive With a P0420 Code?

It depends on how the check engine light is behaving. A steady light usually means the system is running below efficiency — inconvenient, but not immediately dangerous. A blinking or flashing check engine light is a different story: get the vehicle parked in a safe spot and address the issue before driving under normal conditions. Continued driving with a flashing light risks serious engine damage.

Will P0420 Cause Me to Fail My Emissions Test?

Absolutely — and clearing the code won't save you. Even after you erase it, the vehicle's on-board monitoring system will show "not ready" when the shop plugs in their scanner. Most states require all monitors to be complete before a vehicle can pass. You'll need to address the root cause, complete a drive cycle, and confirm the code stays clear.

Overall, a P0420 code is generally not an urgent issue when the light is steady (not flashing) — but ignoring it leads to expensive repairs and eventual mechanical failure. The sooner you diagnose the root cause, the simpler and cheaper the fix.