P0421 Error On 2001 Mazda Protege LX 2.0 4 Cylinder

Would hitting the upper catalytic converter with a rubber mallet free up anything that may be causing the P0421 error and CEL to come on? I need to know if it would do any good or any harm. Thanks from a newbie.

“Warm Up Catalyst Efficiency Below Threshold (Bank 1)”

Forget the rubber mallet…You might try replacing the REAR (downstream) oxygen sensor before you contemplate replacing the catalytic converter…It is the oxygen sensor that is providing the reading that results in the P0421 code and the CEL…

I had both upper and downstream sensors replaced and still P0421.

If both the upstream and downstream O2 sensors were replaced, it pretty much indicates there’s a problem with the catalytic converter. Over time the converter gets contaminated to where it’s efficiency falls off and you get this code.

They’ve come out with a new product that’s suppost to clean the cat to eliminate this code. http://www.catacleanus.com/

This product is so new that I haven’t had a chance to see if it works. So I can’t say if it actually does what it claims.


My mom had a P0420 on her Alero and this is what I found when I was trying to troubleshoot it.

Haven’t tried it yet so I can’t say if it works or not.

A gallon of laquer thinner or some laundry detergent, eh? I want to know if anyone will volunteer as the guinea pig for that one.

Laquer thinner is mostly just acetone…A gallon of acetone in a half-tank of gasoline…You will be lucky if the fuel pump survives that…

There must be a scientific reason that explains how this cleaning process works, the chemical reaction that takes place, but I can’t imagine what it might be…

If the overnight soak in a bucket of detergent works good, imagine how good an overnight soak in a caustic boil-out tank would work…

I think it’s an incorrect assumption that “cleaning” a catalytic converter will somehow restore “like new” function. If a converters working element becomes fouled with contaminants, these contaminants are likely to be fused on metallic compounds built up over a 100K miles of driving and are as insoluble as the ceramic core itself…

“A gallon of laquer thinner or some laundry detergent, eh?”

LOL. I know, it sounds crazy