Free ASE PREP – Catalytic Converter
Another course from my class in January ASE / BAR Specific Diagnostic Course “Smog License”
In the combustion process gasoline combines with Oxygen from the air to create water (H20) and Carbon Dioxide (CO2). Remember gasoline needs oxygen to burn and inside the catalytic converter, the extra air is needed for the oxidation process.
Also, in the combustion process, Carbon Monoxide (CO) combining with Oxygen (O2) creates carbon Dioxide (CO2). This reaction also takes place in the converter when the air/fuel mixture is lean.
Now, to reduce Nox or Nitrous Oxides we need Carbon Monoxide to remove the oxygen from the Nitrous Oxides (Nox) so we can get nitrogen (N2) and Oxygen (O2).
(CO2) Carbon Dioxide is also a byproduct of this reaction.
Note: Replacing a CAT on a vehicle that’s running to lean will not reduce Nox,
Remember for this to happen we need Carbon Monoxide, not air, and with this process, we will have (N2) nitrogen and (O2) Oxygen left over. This process is called reduction, the reaction process that strips o2 from Nox and what’s left over is (o2) Oxygen and (N2) Nitrogen.
The (CAT) catalytic converter job is to clean up the pollution in the exhaust. The noble metal that helps in this process is Platinum, Palladium, for the oxidation and Rodium for the reduction.
There are several different types of converters:
Three-way Cats job is to reduce all three emissions, HC, CO, and Nox. The front bed of the catalytic converter (CAT) contains the Rhodium for the reduction process while the rear bed contains the Platinum and the Palladium for the Oxidation process.
Now with that said there will be a little Platinum and Palladium in front of the Catalytic Converter to help reduce HC and CO.
How Does a Catalytic Converter Work:
One thing you have to know is a three way Cat reduces all three emissions at the same time. In order for this to happen the Cat has to be rich and lean at the same time. How is this possible, the O2 sensor has to be working correctly and fast enough to switch from lean to rich in less than 100 milliseconds, fuel Injection you would like to see 85 milliseconds or less?
On my own personal car,I started to change the catalytic converter (CAT) when the emissions barely pass on the previous smog.
When the emissions standards for Nox’s dropped in California, I thought for sure I would need a Cat.
Before I decide to change it I thought I would check the two O2 sensors, guess what, they were lazy, so I changed the O2 sensors and the Cat worked much better, and my Nox readings dropped and looked much better and the car pass the emissions test.
Now If the air/fuel mixture would stay at 14.7:1, that is 14 pounds of air to 1 pound of fuel or otherwise know has stoichiometry “aka” Balance or good Air/Fuel mixture.
If the air/fuel mixture actually stayed at 14.7:1 and not switch slightly lean the slightly rich, the Cat would not work well in reducing Nox, the air/fuel mixture would have to switch slightly rich to lean and back again in order to control Nox, HC and CO.
In other words, the fuel mixture has to switch slightly rich in order to control Nox then slightly lean to control HC and CO.
In order for this to happen the o2 sensor would have to switch, and switch quickly.
Here is an example:
When the o2 sensor is on the lean side HC and Co will be easily controlled, but Nox is not.
Also, at this time there is also excessive o2 in the system and that o2 is stored in the cerium of the catalytic converter. At this time the O2 sensor is sending a signal back to the computer, “PCM” saying its lean, the computer then responds by increasing fuel command a little, this fuel is then used with the Rhodium in the front of the cat to reduce Nox.
So when should you change the Converter:
So before you change a Catalytic Converter make sure the vehicle has no misfires, the O2 sensor is working correctly and the fuel system is in close loop.
The Cat should be the last thing you change when reducing emissions.
A misfire or a rich running car will kill a new Cat, so fix all other problems prior to replacing the Cat.
Problems resulting from Cats:
A catalytic converter (CAT) coated with or contaminated with oil, coolant, carbon, and antifreeze will cause the cat not to work as well.
A lazy or slow working O2 sensor will also contaminate the converter and cause it not to work as well.
A plugged Cat is a very common problem, when the vehicle misfires and the cat overheats, “sometimes it glows red” it actually melts down and become plugged. When this condition happens you will get a No start condition or a car with no power.
That’s WHY fixing the problem first is so important before you change the CAT!
FREE ASE PREP: Catalytic Converter
Also, remember before changing a catalytic converter there should be no exhaust leaks in front near the converter, especially directly in front of the converter. 1996 and newer, compare the rear O2 sensor
1996 and newer, compare the rear O2 sensor to the front O2 sensor, there should be a 70% difference between the two. The rear should be virtually flat or very little activity.
Remember the rear portion of the Catalytic Converter is the oxidation bed, which oxidizes Hydrocarbons and Carbon Monoxide. Don’t get this confuse with the front bed, which is the reduction bed that reduces Oxides of Nitrogen “Nox”.
Also, remember a three-way catalytic converter lowers all three emissions:
Oxidizes or the process of adding oxygen to reduce Hydrocarbons and Carbon Monoxide
Reduction which is the process of reducing or stripping oxygen away, to reduce Nox