The catalytic converter was introduced nearly 50 years ago because of EPA-ordered cuts in emissions, and in response to those orders, auto industry execs said the EPA’s targets could never be met. But thanks to old-fashioned American ingenuity, they were. What the EPA conceives, our engineers always achieve. Fuel-cell engineer Jonathan Frost once said: "When the U.S. introduced clean air legislation in the 1970s, many engineers said that cleaning up emissions from cars was impossible, but the legislation was passed anyway and new technology was invented in the form of the catalytic converter."
In 1972, Ford President Lee Iacocca said that “if the EPA does not suspend the catalytic converter rule, it will cause Ford to shut down.” He was obviously wrong, but at the time, the prevailing wisdom was that the catalytic converter was a near-impossible concept, overly expensive and inefficient, pegged as an idea that would never, ever work.
The 1970 mandate that auto manufacturers would be required to reduce harmful emissions by 90 percent by the 1975 model year drove Engelhard Industries and Corning Glass to propose the device that would later become the catalytic converter. A “catalyst” foists chemical changes on other elements while resisting any change in itself. The catalytic converter adds the necessary oxygen molecules to CO and HC (the exhaust from rich mixtures), to change those harmful elements into CO2, which is the same thing we breathe out. The exhaust gases flow from the combustion chambers through the catalytic converter’s core, which is a block of ceramic material honeycombed with tiny lengthwise channels, designed to force every cubic millimeter of the gasses into contact with the catalyst material, and that’s where the necessary changes take place.
Lab work finally proved that a catalytic converter would work, but mass-producing them became a new and even more difficult hurdle. Engineers would have to take an abrasive clay mixture and force it through a shaped die at high speed to create the complexities of structure we see in cat-cons today. This process is called “extrusion” and at the time it was very commonly used for creating things like metal pipes and hollow noodles, but nothing anywhere near this complex had ever been attempted. It was a daunting task.
But that wasn’t all. Once that soft block of honeycombed clay emerged from the die, it first had to be cut to the proper length and then heated simultaneously inside and out until it was totally firm. And all this had to be done without distorting those tiny channels or causing the clay to crack, and then they had to find a way to coat all the channels with a layer of very fine platinum particles so that the platinum wouldn’t simply fall off after repeated (and vast) temperature swings. Under normal conditions, a catalytic converter races from ambient temperature to 800° F in 30 seconds, and the temperature of the gases can climb as high as 2,000 degrees. The catalysts on the vehicles I’ve datastreamed lately will normally float between 1000 and 1700°F while driving. In early development, on nearly every prototype, thermal expansion ruined the guts of the converter after just a few drive cycles.
But then it was discovered that samples of clay from one mine in Georgia showed much better heat resistance. This clay turned out to consist of microscopic needle-like units aligned in such a way as to withstand the thermal expansion, and that was the key to making a cat that would last. The cats were off and running, and it has since become our job as technicians to herd them.
Front and rear
Nowadays we’re accustomed to seeing the “light off” cat(s) mounted very near the exhaust manifold(s) to take advantage of the natural heat still present as the exhaust has just made its exit from the combustion event – this front cat is the one sandwiched between the front and rear O2 sensors, and the oxygen that is stored in this converter is extracted from the NOX that is created during the combustion process, leaving N2, which, to quote Bernie Thompson, is the cylinder’s “working fluid” — combustion heats the nitrogen so that it expands against the head of the piston, pushing it down and spinning the crank around. The oxygen extracted from NOX in the front cat is used in the rear cat to handle CO and HC, converting them to harmless CO2, oxygen, and water vapor, which is also created during combustion.
The aft-cat O2 sensor monitors oxygen storage and that sensor’s signal should switch much more sluggishly than the front O2, but if the rear O2 begins to register a problem, the PCM will do what it must with the fuel trim to keep the rear O2 (and the cat) happy, so be ready for that in case you ever see it happening.
So, what damages the cat? Well, engine misfires (caused by no spark or low compression) can lead to overheating and potential meltdown of the substrate. It can be contaminated by silicone sealants (or liquid spray), coolant leaks into the combustion chamber that coat the strata, excessive oil steam blow by that is picked up and processed by the PCV system and sent out the exhaust, high sulfur fuel, and rich fuel mixtures forming carbon deposits. Any of these elements (and some others, like leaded fuel) can quickly coat the catalytic substrate, effectively preventing it from working effectively. Catalysts sometimes just wear out. The brick may break loose inside the shell and dance around in there or chunks may break off the engine side of the cat and rattle around, preventing good exhaust flow. Overheating can also cause cats to break up into dusty, abrasive particles that find their way through the EGR system and into the combustion chamber, a scenario that is an engine killer of the first magnitude. And there are times I’ve seen the vermiculite blanket around the outside of the brick shed material that will clog enough of the passages that exhaust backpressure rises and the engine begins to lose power because it can’t breathe.
The codes and the protective PCM
Just about everybody has seen the P0420 and 430 codes flagged because the rear O2 is reflecting poor oxygen storage, but that doesn’t always mean the cat is bad. The honeycomb will sometimes be coated with one element or another that might well be burned off when the rest of the system is repaired to work right. With that in mind, other DTCs should be carefully considered before condemnation of the cat. Hydrocarbon soot initially cools the cat and makes the O2 sensors sluggish, (skewing the mixture even more) and the soot coating prevents the precious metal substrate from doing its job.
Problems maintaining fuel control can prevent a catalytic converter from working at all. The cat needs a very precise mix of feed gasses to “light off,” and the range expressed in Lambda is very narrow indeed — 1.005-0.995.
One of the first times I encountered PCM cat protection was in the late ’90s when I was working on a Windstar that had all the EGR ports clogged except number four. When EGR was flowing, that one cylinder was getting it all, which created a somewhat mysterious misfire in warm off-idle mode. When I started working on that one, I noticed that when I cracked the throttle, it would begin a very steady misfire on four, and as I was doing my troubleshooting, I noticed that when the misfire was under way, the number four injector would stop clicking, which led me to believe there was a problem in that area. Not knowing that the PCM strategy was shutting the injector down to protect the cat, I did some circuit tracing before I realized that disabling the EGR did away with the skipping, and that’s when I found the reason for the misfire. Ford’s instructors had, at that point, never told me about that strategy in training, but maybe I missed it while I was grabbing a cup of coffee.
If the rear sensor picks up on a consistently dreadful imbalance at the cat’s exit stream and reports it to the PCM, the box might realize that the front O2 is unreliable and fuel trim strategy might be modified based on the rear O2 feedback to protect the cat. This will vary from platform to platform, but the PCM’s concern about the cat’s health is very real.
Once again, all other codes related to air/fuel should be addressed first. MAF, IAT, ECT, rich/lean codes and fuel trim issues would be the focus, along with a careful consideration of the EVAP system and even the condition of the engine oil, both of which can be the source of excessive HC.
| It’s good to have an exhaust gas analyzer on hand. Notice that the display on the left is showing some CO and NOx, along with a 0.992 Lambda reading – there was a problem with this vehicle. The other display shows plenty of CO2 (good), no CO, HC, or NOx, and a perfectly balanced Lambda reading. |
After dealing with the other pertinent codes, make sure you’ve recorded them, erase them, and then start the engine. Hold it at about 2500 to light off the cat, all the while watching front and rear O2 sensors. With a front O2 switching rapidly and a rear O2 trace a lot less active, the converter should be OK. But if the rear O2 sensor is mimicking the front one on a system that is otherwise healthy, the converter is probably ready for the scrap pile, but it’d be wise to dump the codes and complete a drive cycle to see if the converter monitor will flag a problem with the cat before you make your final decision. And there are other considerations.
If the only code is a cat efficiency code, check out the freeze-frame data. If the fuel trim was high when the code was set, it might indicate unmetered air, unreliable MAF readings, or a skewed BARO reading from a speed-density MAP sensor; the fuel pulse will be wrong, and that’ll create a genuine lean condition. The PCM will respond to the O2 sensor(s) lean reporting by dumping extra fuel into the intake to bring things back into balance. If this is happening on just one bank, it’s not likely to be MAF or MAP related and more likely to be something only affecting that bank.
If the fuel trim is double-digit negative, it typically indicates extra unmetered fuel is finding its way into the chambers and the ECM is compensating by subtracting pulse time. This can be fuel injector pressure regulator related, either because the regulator diaphragm is leaking or because it isn’t regulating the pressure at all – shoving 100+ psi of fuel through the nozzles. I’ve seen that more than a few times. And cam timing errors can cause fuel trim anomalies too, so watch out for that.
Visuals and audibles
A cat that is dented or rattling is obviously in need of replacement, because the brick usually can’t take that kind of thing without suffering damage. A discolored converter (“blued and hued,” like some gun parts) is also suspect, because it must get blistering hot to change color that way. It’s also a good idea to get eyes on the front of the brick with a borescope or whatever method you can – if it has clogged combs, is breaking up, or is obviously contaminated, it’ll need replacing, but first you need to find the source of any contamination. You need to find the cause before you attack the effect. Temperature and backpressure tests are a good idea too – it should be hotter at the outlet than at the inlet, and you shouldn’t have more than about a pound of pressure in front of the cat – a good one won’t even have that much. Keep in mind that the temperature test only tells you whether or not the conditions are right for the cat to light. It, alone, does not mean the cat is no good.
| Back in the day, when P0420 codes were new, we’d sometimes see situations where somebody would replace the wrong cat and get a comeback with the same code. The rear cat won’t throw a P0420 code, because there’s no O2 sensor behind it. I saw this a couple of times. The one with the O2s fore and aft is the focal point if everything else is in balance. |
Another common cause of P0420/0430 DTCs are leaky exhaust systems. Even pinhole leaks can suck enough oxygen in to cause a problem. Use your smoke machine to locate the presence of leaks, especially any upstream from the cat and within a foot or two of the backside of the cat.
Then there are those converters that look just fine on the outside but have been gutted by somebody for one reason or another. Back in the 1980s when I was working at a large Ford dealer, some of the mechanics would buy a brand-new truck and immediately have the exhaust system totally re-done – doing away with the cats in the process. For the life of me I’ve never been able to understand why somebody would do that to a new vehicle.
The overarching point is that not every catalyst efficiency code necessarily condemns the cat, although there are many times when that is the case, particularly if everything else is as it should be.
If you’re replacing the cat with anything other than an OEM, make sure you get one that meets the application, and not a “universal” – not all cats are created equal. Back in the 1990s we diagnosed a dead one at the dealer and let the customer talk us into having a replacement put on at a local muffler shop. The replacement converter threw efficiency codes on the first drive cycle. Today, even using one that “looks” right may lead to repeat codes.
