P0421Powertrain
Warm-up catalyst efficiency below limit (bank 1)
Where to start
The catalyst is below par SPECIFICALLY DURING WARM-UP. This is a separate code and a milder one than P0420: often the catalyst is fine and simply takes too long to reach working temperature — because of an oxygen sensor heater, say, or an exhaust leak.
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineEmission controls: catalyst, EGR, fuel vapour
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Catalytic converter»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.