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Making Some Sense of the Catalytic Converter


At its inception, the catalytic converter was never meant for road-going vehicles. Its inventor, Eugen Huadry, initially intended for it to be used in smokestacks and off-highway equipment utilized in mining and warehouse work to reduce emissions output to keep workers safe. After realizing how effective it was, he attempted to use his invention on road-going automobiles. Unfortunately, the higher quality gas used in cars at the time contained tetraethyl lead, and after around ten thousand miles, it would destroy the materials used in the converter. At this point, Eugene gave up because there was no way anyone would give up that sweet, sweet, but very toxic, leaded gas.

This would remain the case until 1970 when the government announced that by 1975 automakers would have to reduce the emissions output of their cars by 90 percent. The automakers claimed the mandate would be impossible to achieve. There was really only one option, and everyone knew it was the catalytic converter. But It was an impossible feat; how would they maintain the high horsepower, high compression engines they were making if they removed lead from gasoline? Because lead was used to avoid engine knock by raising the octane rating of gasoline, and without it, they wouldn’t be able to keep building the engines the same way. But those days were coming to an end and would be gone for a long time as the technology and quality fuel were still a long way out. So automakers slowly started adapting their engines to a new era, and eventually, leaded gas was on its way out.

So now that we have a little background on these magical beasts living in your exhaust system let’s talk about how they work. It’s necessary to understand how something works when you’re trying to diagnose it. First, a catalytic converter is not a filter, though I can understand how someone can come to that conclusion because of how it has been described. Also, since diesel particulate filters came into existence, which is a filter, it’s easy to assume the converter is a filter as well. But a catalytic converter utilizes certain metals, heat, and oxygen as a catalyst to create a reaction, rearranging the gasses that harm the environment into gasses that won’t harm the environment. And because we’re not removing anything, just rearranging things, we can utilize lambda calculators to determine the air-fuel ratio from the measured contents of the exhaust. Whether it’s before or after the converter. This is a vital tool for determining the health of a catalytic converter. Simply put, if the air/fuel ratio isn’t correct, the converter can’t do its job.

Before diagnosing a potentially failed converter, usually associated with the dreaded P0420/430 code on OBD2 vehicles, there can’t be any other problems with the engine or the various components keeping it running right. If you don’t have the proper input into the converter, you won’t have the proper output. Or, in simpler terms, garbage in, garbage out. Things like the mass airflow sensor, oxygen sensors, and ignition system need to be in working order.

Something as simple as an oxygen sensor with a slightly slower-than-normal switching time can throw the air-fuel ratio far enough out that it will lead to what might look like a failed converter on a smog test. A simple way to ensure this would be to check for any other trouble codes stored in the engine control module, including pending codes.

There are a few tests you can do to check the functionality of a catalytic converter. They range in difficulty but starting with the most accessible first just makes sense. This test is based purely on the converter’s temperature and is the most basic test to perform. To start with the car needs to be driven to get the converter up to temperature. Despite what some people think, this doesn’t take a lot of driving; a quick drive on the freeway or a few blocks around town should be enough. If necessary, safely lift the vehicle and place it on jack stands so you can access the converter. Use the infrared thermometer to measure the temperature before and after the converter. The outlet of the converter should be around 150 degrees hotter than the inlet. The chemical reaction taking place inside the converter is what causes the increase in temperature.

Ok, so what if there’s no change in temperature? Well, there are a few factors at play here. The air-fuel ratio has to be correct for the converter to function correctly. Before considering replacing the converter, it would be a good idea to make sure the parts in charge of fuel control are in working order. An upstream oxygen sensor that’s only a few milliseconds off can wreak havoc on the air-fuel ratio, and the converter won’t be able to do its job effectively. I’ve fixed a few late 90s BMWs on the precipice of becoming a gross polluter simply by replacing the upstream oxygen sensors. Older OBD2 diagnostic systems weren’t advanced enough to pick up on minute changes in oxygen sensor switch time. So one could potentially have a failing oxygen sensor and have no idea. There’s something called mode 6 data that can help out here, but that’s another story for another time.

You might have figured out at this point that oxygen sensors play a critical role in the functionality of the converter. They’re a vehicle’s tool to keep the air-fuel ratio correct, keeping the engine and emissions system working as it should. While the front sensor monitors the oxygen content leaving the engine directly and is used to make adjustments to fuel trim, the rear sensor is there to monitor oxygen content exiting the converter. Oxygen storage is key to a functioning converter, and the rear sensor is closely watched by the engine control module to make sure the converter hasn’t failed. This breaks down to the ECM looking at the switch ratio between the front sensor, which is constantly switching, and the rear sensor, which should be switching, at most, around half of what the front sensor is doing. As the ratio moves close to 1:1, the ECM assumes the converter can no longer store oxygen properly and will likely set a P0420/430 trouble code.

Thankfully, anyone with a semi-capable diagnostic scan tool can look at the same oxygen sensor data the engine control module sees. The ability to view live data and compare the oxygen sensor before the converter (upstream) and the oxygen sensor after the converter(downstream) is key to this diagnostic procedure. But first, everything needs to be up to temperature; a few laps around the block or just sitting in the driveway with the engine at 2000 rpm for a few minutes should do it. Ya know, whatever gets the job done. Then you’re going to pull up the live data menu and select the pre-cat and post-cat oxygen sensors, or in some cases, the air-fuel sensor and the post-cat oxygen sensor. Now that the right data is pulled up, it’s time to take a look, but not at idle because some converters can cool down at idle and it will lead to inaccurate results. So, hold the engine at around 2000 rpm or drive and record the data at a steady cruising speed. If you don’t have a scan tool that can record data go grab a friend and have them record the data using their phone. The front sensor is going to switch rapidly from rich (.7 volts or higher) to lean (.2 volts or less), and the rear should be switching very little. For every ten or eleven front switches, there should be less than six for the rear oxygen sensor. If the switch ratio is close to one-to-one it means the converter is no longer capable of storing oxygen and needs replacement.

To be really sure, though, there’s one more test to get some more evidence before dropping a decent chunk of money on a new converter. Let’s be honest, they can be pretty expensive in some cases, and it’s good for that little extra peace of mind. Enter the oxygen storage capacity test. This is the most definitive test to see if a converter has failed, and requires a few extra steps to set up. Once again a scan tool is needed and you’ll need to access the same data as the oxygen sensor test. You’ll need a small propane bottle along with the scan tool. It sounds a little weird I know, but trust me here, it’s important. So make Hank Hill proud and go get some propane and a valve to release it slowly. You can try and use something like carb cleaner but there’s a lot less control over how much you introduce into the intake and you have a higher chance of causing the engine to stumble or stall.

The purpose of the OSC test is to create an artificial rich condition and flood the converter with a rich mixture to remove any remaining stored oxygen. That’s where the propane comes in. Once the scan tool is set up, you need to find a way to introduce the propane into the intake and make sure it’s after the mass airflow sensor. A mass air flow sensor that uses a hot wire poses a bit of a risk. Next, get the scan tool set up with the proper data, ensure the converter is warmed up, and then slowly introduce the propane into the intake and make sure not to let the engine stumble. This should be done for about five or six seconds while watching the data on the scan tool. The oxygen sensors should both be fully rich and, at this point, turn off the propane. The ECM will drive the engine lean and flood the exhaust with oxygen, and almost immediately, the upstream sensor will read as lean as it possibly can. The downstream sensor will stay rich while the converter absorbs as much oxygen as possible, and once it can’t hold anymore, the downstream sensor will go lean as the excess oxygen exits the converter. This should take at least two seconds, but a good converter can sometimes take up to 5 seconds. A failed converter will usually go lean almost immediately because it can no longer absorb oxygen.

The final step is to replace the failed converter with either a new OEM or aftermarket converter. If you live in California, Colorado, or New York you have to select the aftermarket converter that is certified for use in those states. Most manufacturers will specify this on their websites. There are some chemicals or additives on the market that claim to “clean” a failing or failed converter, but from my experience, those don’t work. Once a converter is done and has lost its ability to store oxygen, there’s really no coming back.


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