2002 Mazda Protege catalytic converter

Here learn something about P0421

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.

Symptoms:You 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.

Causes A 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

Possible Solutions: The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady ( 200mV, depends on the vehicle).

If the voltage varies (e.G. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.