When a vehicle gets a P0420 (bank 1, catalytic converter below efficiency) DTC, most owners are not aware that catalytic converters are not actually supposed to be a wear component. If the catalytic converter fails, something else probably created damaging conditions and compromised its function.
What is the catalytic converter? How does it work?
A catalytic converter is a section of the exhaust that uses a catalyst to change hydrocarbons and other pollutants in the exhaust gasses into less hazardess or even harmless chemicals. Its structure is a substrate (usually ceramic) honeycomb that is "washed" (coated) in a precious metal, such as platinum, rhodium, or palladium. A catalytic converter usually is a bulge very near to the engine, sometimes integrated into the exhaust manifold(s), and typically contains multiple sections of the honeycomb catalyst blocks that are 1.5 or so inches long separated by small gaps. It also usually has heat shields because it gets very hot and must retain heat for proper function.
The reason for the use of a honeycomb substrate is to increase surface area. The more exposed metallic surface available, the more catalytic reactions can occur. As the exhaust passes over the catalyst, molecules from it stick to the precious metal and some of the molecular bonds are broken. As more gasses flow over it, other molecules can bump into the held molecules and react to form more acceptable compounds. The catalyst metals are not consumed by these reactions. They help cause more of these reactions by being a sort of work space for reactions to occur on.
One of the functions of the catalytic converter is also to burn off unspent fuel. This is one of the reasons why it must be close to the engine, because heat is necessary to light off unconsumed gasoline. Heat is also a catalyst and helps speed up the reactions. In fact, the engine control module manipulates fuel trims to maintain the proper exhaust temperatures to maximize the efficiency of the catalytic converter, while preventing the temperatures from climbing too high. The engine rapidly cycles between lean and rich conditions to accomplish this; the cycles can be observed on an oscilloscope.
The results of a properly functioning cat are cleaner emissions, a less unpleasant exhaust smell (still gross and unhealthy, don't recommend huffing it), and not getting gross deposits on the bumper above the tailpipe. The engines used in ULEVs such as the Vibe, Matrix, and Corolla probably don't have the last problem so much, they aren't quite as nasty as my Z3, but the exhaust on a vibe without cats smells pretty horrible. The cat also makes the exhaust quieter and supposedly reduces rasp.
What leads to the failure of a catalytic converter?
There are three modes of failure for a catalytic converter: poisoning of the catalyst, meltdown, and collapse of the catalyst blocks.
Collapse of the catalytic blocks occurs via mechanical damage, such as from road debris, oversized speed bumps in parking lots owned by assholes, and excessive vibration from a failed exhaust or engine mount. The collapse of a catalytic converter is recognizable by weak or slow engine rev, sluggish acceleration, quiet exhaust, and an obnoxious rattle. To test for a collapsed catalytic converter, take a mallet and slap the converter with it a few times. If it rattles back at you, it has either collapsed or a heat shield is loose. A collapsed converter can lead to meltdown of the catalyst and engine failure via excessive pressure.
Meltdown occurs when the catalyst gets too dang hot! Blockage and excessive volumes of unburnt gasoline are two causes of high temperatures. If your CEL (check engine light) illuminates and flashes, high volumes of gasoline are exiting the engine due to a malfunction. It indicates a condition that will damage the catalytic converter. Another cause of meltdown is the high temperatures produced by forced induction (turbochargers, superchargers).
Poisoning of the catalyst occurs when a film or coating of an incompatible material is deposited onto the converter metals. Multiple things cause this. I'm not going to get into giving a walk through for diagnosing the systems to identify the faulty components, please use Google for methods to test parts for failure because I'm hardly ever online as I am too involved in my masochistic BMW repair hobby. Don't just buy parts and throw them at the car, that gets extremely expensive and frustrating.
Improper fuel trims, usually rich conditions; things that can cause fuel trims to deviate include:
Worn or fouled spark plugs
A clogged air filter
A thermostat opening too soon or stuck open
A stuck open fuel injector
The wrong temperature range spark plugs
A malfunctioning O2 or air/fuel ratio sensor (rear O2 sensors are sometimes used to control the rich/lean cycling) (some aftermarket sensors are bad enough quality to get incorrect readings)
A malfunctioning, faulty, or poorly manufactured MAF (mass air flow sensor) (NOTE: DO NOT CLEAN YOUR MAF SENSOR WITH ANYTHING EXCEPT MAF SENSOR CLEANER OR YOU WILL DAMAGE THE HOT FILM WIRE. IF I SEE ANOTHER TECHNICIAN USE CARB CLEANER OR BRAKE CLEANER ON AN MAF, I WILL SEND THAT PERSON TO THEIR MOTHER IN A CARDBOARD BOX.)
A faulty coolant temperature sensor
An exhaust leak
Contamination via a worn component or additives or leaded fuel:
Leaded fuel damages catalytic converters and is not healthy for anyone in most any form
Coolant being combusted
Worn piston rings
Worn valve stem seals
Oil entering the combustion chambers in excessive quantities via other sneaky means
Certain oil additives, such as those used in race engine oil or engine break-in oil, or in oil specifying that it should be used only for diesel engines (certain Shell Rotella, for example)
Lastly, a P0420 condition can be caused by the O2 sensor detecting excessive oxygen for reasons not actually caused by cat failure:
An exhaust leak
Excessive lean conditions (look up causes of lean conditions)
Failure of the catalytic converter monitoring O2 sensor itself.
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