P0420: Catalyst system efficiency below threshold (bank 1)

How to read it

P0420 is one of the standard OBD-II trouble codes stored when the check engine light comes on. SAE J2012, the standard that defines the codes, names it “Catalyst System Efficiency Below Threshold Bank 1”; a generic list of OBD-II codes gives the same name.

Also searched as: code P0420, P0420 code.

What the catalyst monitor checks

The federal rule on on-board diagnostics (40 CFR 86.1806-05) requires the system to detect catalyst deterioration or malfunction before it raises hydrocarbon emissions to 1.5 times the standard. SAE J2012 names the two exhaust banks of an engine that has two bank 1 and bank 2, as its diagrams of V6, V8 and V12 engines show.

Can you keep driving with code P0420?

Ford’s F-150 manual says that if the indicator stays on, the vehicle should be serviced at the first available opportunity, and that continued driving with it on can mean higher emissions, lower fuel economy, rougher running and more costly repairs.

How the light goes out

Under the federal rule, the check engine light may go out once the fault has not come back during three trips in a row in which its monitor runs without finding it. The system may erase the stored code if the same code has not been registered again in at least 40 engine warm-up cycles.