[Vettezuki]

I was having some stumbling in the Protege and got this plus another code I forget. They were associated with a dirty MAF. I cleaned the MAF (just a can of spray cleaner) and it runs more or less smoothly, but this code came back. Now goddamnit, my cats were replaced not too long ago!!

Paul, what gives.

[Shaolin Crane]

You made sure you used rubbing alcohol or MAF cleaner right? Carb/brake clean isnt the same and leaves a film that fucks with the sensor.

The other code would probably help in the diagnosis.

[Vettezuki]

MAF cleaner yes. I'm pretty sure the other code was P0171.

[kdracer73]

p0171 is a lean code. Most common causes are vacuum leaks, intake air boot leaks and dirty or messed up air flow sensors.

[Vettezuki]

I'm gonna be a bit pissed if the NEW cats were damaged by a lean condition from a dirty MAF. . .

Warm 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