Catalytic converter deterioration



How do I know if it’s necessary to replace my catalytic converter? If my malfunction indicator light goes on for a day, then turns off, and does this twice in one month’s time, does that indicate a real problem, or would the light stay on continuously if the catalytic converter were really deteriorating at a dangerous level for the engine?


You need to get the error codes read from your car’s computer. Autozone and other auto parts stores will read these codes for free. When they do, post the codes (not their interpretation of the codes) here.

Without those codes, we have no way of knowing what the problem is. For all we know it could be a loose or malfunctioning gas cap.

Usually, the check engine light (CEL) will shine constantly if you have a minor problem, but will flash for severe problems. If you ever see it flash, shut off the car and don’t drive it until it is fixed.


Your MIL illumination means that you need to have the trouble codes downloaded from you car’s computer, nothing more, nothing less.


What year Civic? How many miles?


Get the codes read and post them here.

And meanwhile, don’t stress out. If yuo had a major problem the light would be blinking or staying on solid. Often with an intermittant light like yours is far less serious, often just an EVAP leak or such, and it gets reset.

But don’t procrastinate in getting the codes read. A minor problem can become serious if left unattended.


2002 Civic EX with 87,000 miles


“or would the light stay on continuously if the catalytic converter were really deteriorating at a dangerous level for the engine?”

The ENGINE could not care less…This is strictly an emissions issue…There is no “danger” involved here…You have a nag light on your dash and you can deal with it or not deal with it…


Have the codes read and let us know what they are. They should be in the form [P1234] Many auto part stores will read them for free.


Cat converter will degrade the O2 sensors and cause running problems.

My wife chose to ignore an intermitent Civic’s CEL and ended up with a stalling issue at high speeds. Dealer diagosed as failed cat converter, two O2 sensors but miraculously Honda extended emmissions warranty to 14yrs/150,000 miles and covered everything that ended up a $1700 dealer bill except for a $100 exhaust component.


Why did Honda extend the emissions warranty? My car is only 7 1/2 years old and I’ve gotten every maintenance inspection on time. Shouldn’t the catalytic converter last longer than that?


The code was P0420 catalyst deterioration. The light was on constantly before it turned itself off, then was on constantly one month later when I took it in to be checked. The mechanic didn’t tell me the severity of the deterioration, but the light hasn’t gone back on for over a week.


You had the engine computer scanned, and the scan showed a DTC (Diagnostic Trouble Code) of P0420. This code is for, “Catalyst efficiency below threshold”. Poor, or bad, mechanics/dealers will do the knee-jerk response and just replace the catalytic converter and, possibly, the oxygen sensors.
The catalytic converter, and oxygen sensors, are affected by how efficiently the engine burns its fuel, and is adversely affected by WHAT it burns (oil, antifreeze, excessive amounts of fuel).
The check engine light might not be coming on; but, the condition and efficiency, of the engine, are still affecting the catalytic converter and oxygen sensors.
A good mechanic/dealer will address the operation and condition of the engine, first, before proceeding to the next step in the diagnosis. A poor/bad one won’t.


The catalyst efficiency is measured by the readings of the above and below O2 sensors. Under lean conditions, the catalyst soaks up excess O2. Under rich conditions it dumps it into the exhaust stream. The rear/lower O2 sensor should be (mostly-and within limits) numb to these changes. The catalyst is like an Oxygen “capacitor”. When the lower O2 sensor starts reading more frequent “like states” as the front O2 sensor, the catalysts ability to soak up and dispense O2 is failing. The closer these “co-crossover” events are to zero, the higher the cat efficiency. The higher the number over a given span of time, the lower the cat efficiency.

First you change out the upstream O2 sensor to see if this frequency gets reduced. Lazy fuel management may be the reason for the code. It “overlaps” too long in the rich or the lean state by not reacting quick enough.

I would, however, be prepared (as in save up) for a cat refit at some point in the nearer future.


Thank you to everyone who shared their knowledge about my catalytic converter. I think I have a better idea about what I need to do - first, get a second opinion about the extent of the deterioration, then save money for a new catalytic converter.


Your cat. is covered by a warranty mandated by the federal government (I think Joe Biden runs the program)out to 8 years 80,000 miles,are you in?


Thank you. Unfortunately, although my car is 7 1/2 years old, I have 87,000 miles on it. I’ll have to find out if the warranty is for whichever comes first or last.