P0421 OBD-II Trouble Code
Technical DescriptionWarm Up Catalyst Efficiency Below Threshold (Bank 1)
What does that mean?Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
SymptomsYou will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.
CausesA code P0421 may mean that one or more of the following has happened:
*The catalytic converter is no longer functioning properly
*An oxygen sensor is not reading (functioning) properly
*A sparkplug is fouled up
My $ is on one of the last 2 (or so I would hope)