How to Tell if Catalytic Converter Is Going Bad
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More than 90% of cars today are powered by internal combustion engines (ICE)—and until electric vehicles completely take over, this type of engine is likely here to stay. However, it isn't ideal as it can create harmful toxic chemical compounds that have a negative environmental impact.
Catalytic converters help combat these harmful emissions by employing precious metals to convert toxic gases into non-threatening substances. Due to increasing global environmental awareness, this exhaust system component, also referred to as a "cat" for short, is considered one of the most important inventions in the history of automobiles.
Signs of a Clogged Catalytic Converter
A clogged catalytic converter is different from a cat that is worn out or damaged chemically. A clogged catalytic converter will present symptoms, while a chemically failed cat con that's not clogged typically just illuminates the check engine light or malfunction indicator lamp (MIL). Also, if the cat con that just fails chemically but doesn't clog is not the light-off cat con (the cat con closest to the engine with an O2 sensor keeping tabs on it), then that failed cat con won't cause any symptoms at all.
In other words: not all bad catalytic converters are clogged catalytic converters. A catalytic converter can lose its efficiency without being clogged.
But the biggest problem with a clogged cat is that it can create excessive exhaust backpressure. Basically, because the clogged cat restricts exhaust flow, it prevents the engine from "breathing" properly—which can lead to a wide range of engine performance problems.
Here are some common signs of a clogged catalytic converter:
Poor fuel economy and engine performance
A clogged catalytic converter prevents your engine from breathing properly. As a result your car may experience a lack of acceleration and overall performance. You may also notice the fuel economy drop.
Difficulty starting the engine and stalling
Excessive exhaust back pressure can choke the engine, causing it to stall. In the case of extreme blockage, you may find your engine starting and idling for a couple of minutes, only to die shortly afterward. As a matter of fact, A cat can be clogged so badly that the engine won't even start. The caveat is that if you've continued to drive it until it reaches this point, you've probably already noticed poor fuel economy and engine performance unless you just weren't paying attention.
Illuminated malfunction indicator lamp
The check engine light is your car's way of telling you, "I need to see a doctor." It's basically warning you about certain issues that are causing your engine to perform poorly and inefficiently. The check engine light/ MIL may not illuminate if the catalyst is only beginning to clog, so don't think the catalyst can't be clogged if you have reduced power but the light doesn't illuminate.
Failed emission test
A clogged cat can trigger the check engine light – and that will cause you to fail an emissions test in most locations. Furthermore, if the clogged cat has come apart inside, your car is unlikely to pass a tailpipe emissions test (if one is required).
As previously mentioned, clogs can even cause the catalyst materials to break into pieces. If you suspect that you've reached this level of damage, give the cat a tap and listen for rattling noises. What's even worse is that on some engines, if the "honeycomb" part of the catalyst begins to break up, particles of the cat (because of the dynamics of exhaust pulses) can enter the cylinders and destroy the engine. 20xx Nissan Altimas are prone to this kind of damage.
Other Ways of Diagnosing Catalytic Converter Failure
Using a Vacuum Gauge
One quick and easy test is to connect a vacuum gauge to a manifold vacuum port (like the one feeding the brake booster). Idling on a healthy engine, the gauge should read 18-22 inches of vacuum idling in park or neutral. Wear safety glasses and remain clear of moving parts while you have an assistant raise the engine speed (still in park or neutral). If you can operate the throttle yourself from under the hood (be careful) that works too.
On a healthy engine, the vacuum should dive very briefly and then recover to a slightly higher reading than where it was at idle. If the vacuum drops with the engine speed raised and stays lower than it was with the engine idling, the catalyst is probably clogged. If raising the engine speed a little more lowers the needle even further, it's more certain. At this point, move to the next step below.
If you have the tools to do it (this can be tough on some vehicles), One of the first things an experienced mechanic will do to trace a clogged converter is they will temporarily remove the oxygen sensor or unbolt the exhaust downpipe. If engine performance improves with the sensor or the exhaust removed, chances are high that the catalytic converter is the component that's causing the problem. Of course, the engine is extremely noisy when the sensor is out or the exhaust is removed and if you drive the vehicle with the O2 sensor removed or the exhaust disconnected, the heat can be intense enough to cause damage to nearby components, so don't run the engine too long or too much with the O2 removed or the exhaust disconnected.
You may also check the structure of the converter for any impact damage caused by running over road debris, but any road debris that has damaged the converter usually causes other noticeable damage.
Testing for Backpressure
There's another way of confirming your suspicion—by measuring the backpressure. To do this, you're going to need a low pressure gauge with a scale that reads up to 15 PSI. You may also opt for a basic backpressure test kit (available from online suppliers for $20-$30). These kits typically require removal of the O2 sensor.
If you want a more accurate reading, you can use a digital manometer ($40-$50 online), or a pressure gauge with a variety of units of measurement. Any of these require some kind of connection to the exhaust in front of the converter being tested.
If your engine has a secondary air injection system, you can disconnect the check valve and install your pressure gauge. You may get a more accurate result if you connect the check valve to the exhaust system before the converter. But note that connecting to the check valve (which is usually a ⅝ hose connection) would require building some type of adapter).
You could also drill a hole along the exhaust pipe (just ahead of the catalytic converter) where you can attach the pressure gauge. This is your best option if you have an older car and are worried about damaging the O2 sensor. If you drill a ⅛ hole you can simply hold a square cut hose up to the hole while an assistant increases engine speed. After the test you can install a self-tapping screw or, better yet, a blind pop rivet in the ⅛ hole.
The backpressure in an idling vehicle can vary depending on its year, make, and model. Typically, an engine at idle should have 1.5 PSI at most. Of course, some engines can go way higher, but the rule of thumb is to have 1.5 PSI and below. Less than 1 psi is optimum.
When you rev the vehicle at 2,000 RPM and it remains steady at or below 3 PSI, then you likely don't have issues in your catalytic converter. But if it fluctuates or increases beyond 3 PSI despite having a steady RPM, it's a good indication that backpressure is building up.
Revving up the engine will increase the pressure, which is normal. However, an abnormal increase in pressure at a steady RPM can indicate possible backpressure.
Be aware that the "brick" in a catalytic converter can, in some cases, break loose enough that it can intermittently turn sideways so that the backpressure problem isn't always present, but in that case, whacking the converter with a rubber hammer will create a rattle that will pinpoint the loose brick.
Other Ways to Check Back Pressure
It's worth noting that you can also check back pressure with a vacuum gauge (not a pressure gauge). Connecting a gauge to a source of manifold vacuum is much easier than connecting a gauge to the exhaust.
Additionally, it may be possible to get an idea of back pressure by monitoring a scan tool and looking at parameters, such as manifold absolute pressure (MAP) and calculated load.
When to Replace or Repair Your Catalytic Converter
A clogged catalytic converter should be replaced. Although there are products on the market that claim to be catalytic converter cleaners, you can't believe everything you read.
Actually, replacing a clogged cat is the only way to fix the problem. Removing the catalytic converter and replacing it with a pipe is a breach of federal law, even in states that don't do emissions testing. Don't go there.
What Does the Catalytic Converter Do?
A catalytic converter reduces the emissions from the engine's exhaust by means of a chemical reaction. If you split the assembly in half, you'll have two primary sections. Most vehicles have two converters – one near the engine (the light-off cat that handles NOx). This one is monitored for efficiency by a downstream O2 sensor. The cat that handles the HC and CO is the rearmost cat (usually a separate unit from the front one on newer cars) and is not monitored by a downstream O2.
The first one is a catalyst with a combination of platinum and rhodium. It works by breaking down oxides of nitrogen (NOx) into nitrogen and oxygen molecules.
As the exhaust gas travels further, it passes through a second catalyst, which is a combination of platinum and palladium. Here is where two-way oxidation takes place—carbon monoxide (CO) and hydrocarbons (HC) are broken down into less harmful molecules such as carbon dioxide (CO2) and water (H2O). A properly balanced fuel mix will produce mostly CO2 and water. A slightly rich mixture will produce CO (not enough O2 to go around) and a leaking injector or VERY rich mixture will produce high HC (hydrocarbon – unburned fuel) in the exhaust, which shows up as soot.
NOx is produced when combustion chamber temperatures exceed 2500 degrees, because most of the atmosphere in the mix (78%) is nitrogen, which is superheated by the combustion event to drive the piston down. Above 2500 degrees, nitrogen molecules bond to O2 molecules in various different compounds – that's why it's referred to as NOx (x can be any one of several different numbers).
The oxygen (O2) molecules from the first catalytic conversion bond with CO molecules and form CO2. The hydrogen and carbon molecules from the hydrocarbon compound split to bond with oxygen and form CO2 and H2O.
After the whole process, the now-less-harmful exhaust gas travels until it reaches the end of the tailpipe, where it's dispersed into the atmosphere.
For your car to pass the emissions test, the catalytic converter should be in optimal condition.
What Can Go Wrong With Your Catalytic Converter?
Damaged catalytic converters not only cause your vehicle to fail an emissions test, but it can also damage neighboring parts, resulting in more expensive repairs.
There are multiple reasons why your cat might fail prematurely, including:
- Contamination from substances such as antifreeze and oil
- Dents that can damage the catalysts
- Engine performance problems
Foreign substances, such as coolant and oil, can get into the exhaust due to engine problems upstream. Such contaminants can easily destroy your catalytic converter.The same goes for dents, as a blockage in the catalysts can also be a result of physical damage.
Engine performance problems can damage your catalytic converter, as well. For example, an engine misfire or an improper air/fuel mixture can cause the cat to overheat. And that can lead to its early demise.
Good aftermarket catalytic converters, which are available at a lower price than OEM converters, will help make sure you don't get a new catalyst efficiency code.
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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.
How to Tell if Catalytic Converter Is Going Bad
Source: https://www.carparts.com/blog/how-to-tell-if-the-catalytic-converter-is-clogged/